[RFC PATCH 0/1] ext4: Try to better reuse recently freed space

[RFC PATCH 0/1] ext4: Try to better reuse recently freed space

[Lukas Czerner]

Hi all,

as I just recently discovered here http://www.ogris.de/blkalloc/ ext4 have
some unexpected allocation strategies which can cause some problems in
certain scenarios (see the 1/1 patch).

This is my attempt to fix this, however I think that this will need some
discussion because it changes some of the block allocator heuristic
quite significantly and I would like to make sure that it will not hurt
our performance in some widely used workloads. Eric, could you run your
performance testing with your test suite to see how it performs ?

Also, I think that the old behaviour might be quite helpful for SSD because
we're not overwriting existing blocks but rather trying to use new one, so
the firmware should have easier job. However I do not know how significant
impact if at all it might have. If we find out that this is helpful for
SSD we might make this conditional depending on the type of the device.

Which brings me to the other problem. Since the original behaviour is really
bad for thinly provisioned devices we would like to avoid using it even if
the underlying storage is SSD, however IIRC there is no way to distinguish
this from within the kernel.

See the comparison between the old and new allocator behaviour
http://people.redhat.com/lczerner/allocator/

Comments and testing is welcomed.

Thanks!
-Lukas

[RFC PATCH 1/1] ext4: Try to better reuse recently freed space

[Lukas Czerner]

Currently if the block allocator can not find the goal to allocate we
would use global goal for stream allocation. However the global goal
(s_mb_last_group and s_mb_last_start) will move further every time such
allocation appears and never move backwards.

This causes several problems in certain scenarios:

- the goal will move further and further preventing us from reusing
  space which might have been freed since then. This is ok from the file
  system point of view because we will reuse that space eventually,
  however we're allocating block from slower parts of the spinning disk
  even though it might not be necessary.
- The above also causes more serious problem for example for thinly
  provisioned storage (sparse images backed storage as well), because
  instead of reusing blocks which are already provisioned we would try
  to use new blocks. This would unnecessarily drain storage free blocks
  pool.
- This will also cause blocks to be allocated further from the given
  goal than it's necessary. Consider for example truncating, or removing
  and rewriting the file in the loop. This workload will never reuse
  freed blocks until we continually claim and free all the block in the
  file system.

Note that file systems like xfs, ext3, or btrfs does not have this
problem. This is simply caused by the notion of global pool.

Fix this by changing the global goal to be goal per inode. This will
allow us to invalidate the goal every time the inode has been truncated,
or newly created, so in those cases we would try to use the proper more
specific goal which is based on inode position.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
---
 fs/ext4/ext4.h    |  7 ++++---
 fs/ext4/inode.c   |  8 ++++++++
 fs/ext4/mballoc.c | 20 ++++++++------------
 3 files changed, 20 insertions(+), 15 deletions(-)

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index 6ed348d..4dffa92 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -917,6 +917,10 @@ struct ext4_inode_info {
 
 	/* Precomputed uuid+inum+igen checksum for seeding inode checksums */
 	__u32 i_csum_seed;
+
+	/* where last allocation was done - for stream allocation */
+	unsigned long i_last_group;
+	unsigned long i_last_start;
 };
 
 /*
@@ -1242,9 +1246,6 @@ struct ext4_sb_info {
 	unsigned int s_mb_order2_reqs;
 	unsigned int s_mb_group_prealloc;
 	unsigned int s_max_dir_size_kb;
-	/* where last allocation was done - for stream allocation */
-	unsigned long s_mb_last_group;
-	unsigned long s_mb_last_start;
 
 	/* stats for buddy allocator */
 	atomic_t s_bal_reqs;	/* number of reqs with len > 1 */
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 0188e65..07d0434 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -3702,6 +3702,10 @@ void ext4_truncate(struct inode *inode)
 	else
 		ext4_ind_truncate(handle, inode);
 
+	/* Invalidate last allocation counters */
+	ei->i_last_group = UINT_MAX;
+	ei->i_last_start = UINT_MAX;
+
 	up_write(&ei->i_data_sem);
 
 	if (IS_SYNC(inode))
@@ -4060,6 +4064,10 @@ struct inode *ext4_iget(struct super_block *sb, unsigned long ino)
 	inode->i_generation = le32_to_cpu(raw_inode->i_generation);
 	ei->i_block_group = iloc.block_group;
 	ei->i_last_alloc_group = ~0;
+
+	/* Invalidate last allocation counters */
+	ei->i_last_group = UINT_MAX;
+	ei->i_last_start = UINT_MAX;
 	/*
 	 * NOTE! The in-memory inode i_data array is in little-endian order
 	 * even on big-endian machines: we do NOT byteswap the block numbers!
diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index a9ff5e5..6c23666 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -1591,7 +1591,6 @@ static int mb_mark_used(struct ext4_buddy *e4b, struct ext4_free_extent *ex)
 static void ext4_mb_use_best_found(struct ext4_allocation_context *ac,
 					struct ext4_buddy *e4b)
 {
-	struct ext4_sb_info *sbi = EXT4_SB(ac->ac_sb);
 	int ret;
 
 	BUG_ON(ac->ac_b_ex.fe_group != e4b->bd_group);
@@ -1622,10 +1621,8 @@ static void ext4_mb_use_best_found(struct ext4_allocation_context *ac,
 	get_page(ac->ac_buddy_page);
 	/* store last allocated for subsequent stream allocation */
 	if (ac->ac_flags & EXT4_MB_STREAM_ALLOC) {
-		spin_lock(&sbi->s_md_lock);
-		sbi->s_mb_last_group = ac->ac_f_ex.fe_group;
-		sbi->s_mb_last_start = ac->ac_f_ex.fe_start;
-		spin_unlock(&sbi->s_md_lock);
+		EXT4_I(ac->ac_inode)->i_last_group = ac->ac_f_ex.fe_group;
+		EXT4_I(ac->ac_inode)->i_last_start = ac->ac_f_ex.fe_start;
 	}
 }
 
@@ -2080,13 +2077,12 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
 			ac->ac_2order = i - 1;
 	}
 
-	/* if stream allocation is enabled, use global goal */
-	if (ac->ac_flags & EXT4_MB_STREAM_ALLOC) {
-		/* TBD: may be hot point */
-		spin_lock(&sbi->s_md_lock);
-		ac->ac_g_ex.fe_group = sbi->s_mb_last_group;
-		ac->ac_g_ex.fe_start = sbi->s_mb_last_start;
-		spin_unlock(&sbi->s_md_lock);
+	/* if stream allocation is enabled and per inode goal is
+	 * set, use it */
+	if ((ac->ac_flags & EXT4_MB_STREAM_ALLOC) &&
+	   (EXT4_I(ac->ac_inode)->i_last_start != UINT_MAX)) {
+		ac->ac_g_ex.fe_group = EXT4_I(ac->ac_inode)->i_last_group;
+		ac->ac_g_ex.fe_start = EXT4_I(ac->ac_inode)->i_last_start;
 	}
 
 	/* Let's just scan groups to find more-less suitable blocks */
-- 
1.8.3.1

Re: [RFC PATCH 1/1] ext4: Try to better reuse recently freed space

[Jan Kara]

On Thu 04-07-13 11:11:54, Lukas Czerner wrote:
> Currently if the block allocator can not find the goal to allocate we
> would use global goal for stream allocation. However the global goal
> (s_mb_last_group and s_mb_last_start) will move further every time such
> allocation appears and never move backwards.
> 
> This causes several problems in certain scenarios:
> 
> - the goal will move further and further preventing us from reusing
>   space which might have been freed since then. This is ok from the file
>   system point of view because we will reuse that space eventually,
>   however we're allocating block from slower parts of the spinning disk
>   even though it might not be necessary.
> - The above also causes more serious problem for example for thinly
>   provisioned storage (sparse images backed storage as well), because
>   instead of reusing blocks which are already provisioned we would try
>   to use new blocks. This would unnecessarily drain storage free blocks
>   pool.
> - This will also cause blocks to be allocated further from the given
>   goal than it's necessary. Consider for example truncating, or removing
>   and rewriting the file in the loop. This workload will never reuse
>   freed blocks until we continually claim and free all the block in the
>   file system.
> 
> Note that file systems like xfs, ext3, or btrfs does not have this
> problem. This is simply caused by the notion of global pool.
> 
> Fix this by changing the global goal to be goal per inode. This will
> allow us to invalidate the goal every time the inode has been truncated,
> or newly created, so in those cases we would try to use the proper more
> specific goal which is based on inode position.
  In principle the patch looks fine to me. Just minor style nit below:

> 
> Signed-off-by: Lukas Czerner <lczerner@redhat.com>
> ---
>  fs/ext4/ext4.h    |  7 ++++---
>  fs/ext4/inode.c   |  8 ++++++++
>  fs/ext4/mballoc.c | 20 ++++++++------------
>  3 files changed, 20 insertions(+), 15 deletions(-)
> 
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index 6ed348d..4dffa92 100644
> --- a/fs/ext4/ext4.h
> +++ b/fs/ext4/ext4.h
> @@ -917,6 +917,10 @@ struct ext4_inode_info {
>  
>  	/* Precomputed uuid+inum+igen checksum for seeding inode checksums */
>  	__u32 i_csum_seed;
> +
> +	/* where last allocation was done - for stream allocation */
> +	unsigned long i_last_group;
> +	unsigned long i_last_start;
  Can we use proper types here please? I think they should be
ext4_grpblk_t and ext4_group_t. And maybe we should define something like
EXT4_INVAL_GRPNO and use that instead of UINT_MAX.

								Honza
>  };
>  
>  /*
> @@ -1242,9 +1246,6 @@ struct ext4_sb_info {
>  	unsigned int s_mb_order2_reqs;
>  	unsigned int s_mb_group_prealloc;
>  	unsigned int s_max_dir_size_kb;
> -	/* where last allocation was done - for stream allocation */
> -	unsigned long s_mb_last_group;
> -	unsigned long s_mb_last_start;
>  
>  	/* stats for buddy allocator */
>  	atomic_t s_bal_reqs;	/* number of reqs with len > 1 */
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index 0188e65..07d0434 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -3702,6 +3702,10 @@ void ext4_truncate(struct inode *inode)
>  	else
>  		ext4_ind_truncate(handle, inode);
>  
> +	/* Invalidate last allocation counters */
> +	ei->i_last_group = UINT_MAX;
> +	ei->i_last_start = UINT_MAX;
> +
  And 
>  	up_write(&ei->i_data_sem);
>  
>  	if (IS_SYNC(inode))
> @@ -4060,6 +4064,10 @@ struct inode *ext4_iget(struct super_block *sb, unsigned long ino)
>  	inode->i_generation = le32_to_cpu(raw_inode->i_generation);
>  	ei->i_block_group = iloc.block_group;
>  	ei->i_last_alloc_group = ~0;
> +
> +	/* Invalidate last allocation counters */
> +	ei->i_last_group = UINT_MAX;
> +	ei->i_last_start = UINT_MAX;
>  	/*
>  	 * NOTE! The in-memory inode i_data array is in little-endian order
>  	 * even on big-endian machines: we do NOT byteswap the block numbers!
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index a9ff5e5..6c23666 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -1591,7 +1591,6 @@ static int mb_mark_used(struct ext4_buddy *e4b, struct ext4_free_extent *ex)
>  static void ext4_mb_use_best_found(struct ext4_allocation_context *ac,
>  					struct ext4_buddy *e4b)
>  {
> -	struct ext4_sb_info *sbi = EXT4_SB(ac->ac_sb);
>  	int ret;
>  
>  	BUG_ON(ac->ac_b_ex.fe_group != e4b->bd_group);
> @@ -1622,10 +1621,8 @@ static void ext4_mb_use_best_found(struct ext4_allocation_context *ac,
>  	get_page(ac->ac_buddy_page);
>  	/* store last allocated for subsequent stream allocation */
>  	if (ac->ac_flags & EXT4_MB_STREAM_ALLOC) {
> -		spin_lock(&sbi->s_md_lock);
> -		sbi->s_mb_last_group = ac->ac_f_ex.fe_group;
> -		sbi->s_mb_last_start = ac->ac_f_ex.fe_start;
> -		spin_unlock(&sbi->s_md_lock);
> +		EXT4_I(ac->ac_inode)->i_last_group = ac->ac_f_ex.fe_group;
> +		EXT4_I(ac->ac_inode)->i_last_start = ac->ac_f_ex.fe_start;
>  	}
>  }
>  
> @@ -2080,13 +2077,12 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
>  			ac->ac_2order = i - 1;
>  	}
>  
> -	/* if stream allocation is enabled, use global goal */
> -	if (ac->ac_flags & EXT4_MB_STREAM_ALLOC) {
> -		/* TBD: may be hot point */
> -		spin_lock(&sbi->s_md_lock);
> -		ac->ac_g_ex.fe_group = sbi->s_mb_last_group;
> -		ac->ac_g_ex.fe_start = sbi->s_mb_last_start;
> -		spin_unlock(&sbi->s_md_lock);
> +	/* if stream allocation is enabled and per inode goal is
> +	 * set, use it */
> +	if ((ac->ac_flags & EXT4_MB_STREAM_ALLOC) &&
> +	   (EXT4_I(ac->ac_inode)->i_last_start != UINT_MAX)) {
> +		ac->ac_g_ex.fe_group = EXT4_I(ac->ac_inode)->i_last_group;
> +		ac->ac_g_ex.fe_start = EXT4_I(ac->ac_inode)->i_last_start;
>  	}
>  
>  	/* Let's just scan groups to find more-less suitable blocks */
> -- 
> 1.8.3.1
> 
> --
> To unsubscribe from this list: send the line "unsubscribe linux-ext4" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR

Re: [RFC PATCH 1/1] ext4: Try to better reuse recently freed space

[Lukáš Czerner]

On Thu, 4 Jul 2013, Jan Kara wrote:

> Date: Thu, 4 Jul 2013 17:09:05 +0200
> From: Jan Kara <jack@suse.cz>
> To: Lukas Czerner <lczerner@redhat.com>
> Cc: linux-ext4@vger.kernel.org, enwlinux@gmail.com,
>     Jose_Mario_Gallegos@Dell.com, jordan_hargrave@Dell.com,
>     rwheeler@redhat.com
> Subject: Re: [RFC PATCH 1/1] ext4: Try to better reuse recently freed space
> 
> On Thu 04-07-13 11:11:54, Lukas Czerner wrote:
> > Currently if the block allocator can not find the goal to allocate we
> > would use global goal for stream allocation. However the global goal
> > (s_mb_last_group and s_mb_last_start) will move further every time such
> > allocation appears and never move backwards.
> > 
> > This causes several problems in certain scenarios:
> > 
> > - the goal will move further and further preventing us from reusing
> >   space which might have been freed since then. This is ok from the file
> >   system point of view because we will reuse that space eventually,
> >   however we're allocating block from slower parts of the spinning disk
> >   even though it might not be necessary.
> > - The above also causes more serious problem for example for thinly
> >   provisioned storage (sparse images backed storage as well), because
> >   instead of reusing blocks which are already provisioned we would try
> >   to use new blocks. This would unnecessarily drain storage free blocks
> >   pool.
> > - This will also cause blocks to be allocated further from the given
> >   goal than it's necessary. Consider for example truncating, or removing
> >   and rewriting the file in the loop. This workload will never reuse
> >   freed blocks until we continually claim and free all the block in the
> >   file system.
> > 
> > Note that file systems like xfs, ext3, or btrfs does not have this
> > problem. This is simply caused by the notion of global pool.
> > 
> > Fix this by changing the global goal to be goal per inode. This will
> > allow us to invalidate the goal every time the inode has been truncated,
> > or newly created, so in those cases we would try to use the proper more
> > specific goal which is based on inode position.
>   In principle the patch looks fine to me. Just minor style nit below:
> 
> > 
> > Signed-off-by: Lukas Czerner <lczerner@redhat.com>
> > ---
> >  fs/ext4/ext4.h    |  7 ++++---
> >  fs/ext4/inode.c   |  8 ++++++++
> >  fs/ext4/mballoc.c | 20 ++++++++------------
> >  3 files changed, 20 insertions(+), 15 deletions(-)
> > 
> > diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> > index 6ed348d..4dffa92 100644
> > --- a/fs/ext4/ext4.h
> > +++ b/fs/ext4/ext4.h
> > @@ -917,6 +917,10 @@ struct ext4_inode_info {
> >  
> >  	/* Precomputed uuid+inum+igen checksum for seeding inode checksums */
> >  	__u32 i_csum_seed;
> > +
> > +	/* where last allocation was done - for stream allocation */
> > +	unsigned long i_last_group;
> > +	unsigned long i_last_start;
>   Can we use proper types here please? I think they should be
> ext4_grpblk_t and ext4_group_t. And maybe we should define something like
> EXT4_INVAL_GRPNO and use that instead of UINT_MAX.

Sure, it'll look better with named types and defined invalidate
value. I'll resend the patch soon.

Thanks!
-Lukas

> 
> 								Honza
> >  };
> >  
> >  /*
> > @@ -1242,9 +1246,6 @@ struct ext4_sb_info {
> >  	unsigned int s_mb_order2_reqs;
> >  	unsigned int s_mb_group_prealloc;
> >  	unsigned int s_max_dir_size_kb;
> > -	/* where last allocation was done - for stream allocation */
> > -	unsigned long s_mb_last_group;
> > -	unsigned long s_mb_last_start;
> >  
> >  	/* stats for buddy allocator */
> >  	atomic_t s_bal_reqs;	/* number of reqs with len > 1 */
> > diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> > index 0188e65..07d0434 100644
> > --- a/fs/ext4/inode.c
> > +++ b/fs/ext4/inode.c
> > @@ -3702,6 +3702,10 @@ void ext4_truncate(struct inode *inode)
> >  	else
> >  		ext4_ind_truncate(handle, inode);
> >  
> > +	/* Invalidate last allocation counters */
> > +	ei->i_last_group = UINT_MAX;
> > +	ei->i_last_start = UINT_MAX;
> > +
>   And 
> >  	up_write(&ei->i_data_sem);
> >  
> >  	if (IS_SYNC(inode))
> > @@ -4060,6 +4064,10 @@ struct inode *ext4_iget(struct super_block *sb, unsigned long ino)
> >  	inode->i_generation = le32_to_cpu(raw_inode->i_generation);
> >  	ei->i_block_group = iloc.block_group;
> >  	ei->i_last_alloc_group = ~0;
> > +
> > +	/* Invalidate last allocation counters */
> > +	ei->i_last_group = UINT_MAX;
> > +	ei->i_last_start = UINT_MAX;
> >  	/*
> >  	 * NOTE! The in-memory inode i_data array is in little-endian order
> >  	 * even on big-endian machines: we do NOT byteswap the block numbers!
> > diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> > index a9ff5e5..6c23666 100644
> > --- a/fs/ext4/mballoc.c
> > +++ b/fs/ext4/mballoc.c
> > @@ -1591,7 +1591,6 @@ static int mb_mark_used(struct ext4_buddy *e4b, struct ext4_free_extent *ex)
> >  static void ext4_mb_use_best_found(struct ext4_allocation_context *ac,
> >  					struct ext4_buddy *e4b)
> >  {
> > -	struct ext4_sb_info *sbi = EXT4_SB(ac->ac_sb);
> >  	int ret;
> >  
> >  	BUG_ON(ac->ac_b_ex.fe_group != e4b->bd_group);
> > @@ -1622,10 +1621,8 @@ static void ext4_mb_use_best_found(struct ext4_allocation_context *ac,
> >  	get_page(ac->ac_buddy_page);
> >  	/* store last allocated for subsequent stream allocation */
> >  	if (ac->ac_flags & EXT4_MB_STREAM_ALLOC) {
> > -		spin_lock(&sbi->s_md_lock);
> > -		sbi->s_mb_last_group = ac->ac_f_ex.fe_group;
> > -		sbi->s_mb_last_start = ac->ac_f_ex.fe_start;
> > -		spin_unlock(&sbi->s_md_lock);
> > +		EXT4_I(ac->ac_inode)->i_last_group = ac->ac_f_ex.fe_group;
> > +		EXT4_I(ac->ac_inode)->i_last_start = ac->ac_f_ex.fe_start;
> >  	}
> >  }
> >  
> > @@ -2080,13 +2077,12 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
> >  			ac->ac_2order = i - 1;
> >  	}
> >  
> > -	/* if stream allocation is enabled, use global goal */
> > -	if (ac->ac_flags & EXT4_MB_STREAM_ALLOC) {
> > -		/* TBD: may be hot point */
> > -		spin_lock(&sbi->s_md_lock);
> > -		ac->ac_g_ex.fe_group = sbi->s_mb_last_group;
> > -		ac->ac_g_ex.fe_start = sbi->s_mb_last_start;
> > -		spin_unlock(&sbi->s_md_lock);
> > +	/* if stream allocation is enabled and per inode goal is
> > +	 * set, use it */
> > +	if ((ac->ac_flags & EXT4_MB_STREAM_ALLOC) &&
> > +	   (EXT4_I(ac->ac_inode)->i_last_start != UINT_MAX)) {
> > +		ac->ac_g_ex.fe_group = EXT4_I(ac->ac_inode)->i_last_group;
> > +		ac->ac_g_ex.fe_start = EXT4_I(ac->ac_inode)->i_last_start;
> >  	}
> >  
> >  	/* Let's just scan groups to find more-less suitable blocks */
> > -- 
> > 1.8.3.1
> > 
> > --
> > To unsubscribe from this list: send the line "unsubscribe linux-ext4" in
> > the body of a message to majordomo@vger.kernel.org
> > More majordomo info at  http://vger.kernel.org/majordomo-info.html
> 

RE: [RFC PATCH 1/1] ext4: Try to better reuse recently freed space

[Jose_Mario_Gallegos]

Thank you very much for looking into this issue Lukas!

Ric,
What are the odds of getting a patch as a hot fix or beta for a proof of concept?
The deadline is 7/15/2013 and this may eliminate the biggest remaining issue.

Thanks again and best regards,

Mario

> -----Original Message-----
> From: Lukáš Czerner [mailto:lczerner@redhat.com]
> Sent: Thursday, July 04, 2013 10:33 AM
> To: Jan Kara
> Cc: linux-ext4@vger.kernel.org; enwlinux@gmail.com; Gallegos, Mario;
> Hargrave, Jordan; rwheeler@redhat.com
> Subject: Re: [RFC PATCH 1/1] ext4: Try to better reuse recently freed space
> 
> On Thu, 4 Jul 2013, Jan Kara wrote:
> 
> > Date: Thu, 4 Jul 2013 17:09:05 +0200
> > From: Jan Kara <jack@suse.cz>
> > To: Lukas Czerner <lczerner@redhat.com>
> > Cc: linux-ext4@vger.kernel.org, enwlinux@gmail.com,
> >     Jose_Mario_Gallegos@Dell.com, jordan_hargrave@Dell.com,
> >     rwheeler@redhat.com
> > Subject: Re: [RFC PATCH 1/1] ext4: Try to better reuse recently freed
> > space
> >
> > On Thu 04-07-13 11:11:54, Lukas Czerner wrote:
> > > Currently if the block allocator can not find the goal to allocate
> > > we would use global goal for stream allocation. However the global
> > > goal (s_mb_last_group and s_mb_last_start) will move further every
> > > time such allocation appears and never move backwards.
> > >
> > > This causes several problems in certain scenarios:
> > >
> > > - the goal will move further and further preventing us from reusing
> > >   space which might have been freed since then. This is ok from the file
> > >   system point of view because we will reuse that space eventually,
> > >   however we're allocating block from slower parts of the spinning disk
> > >   even though it might not be necessary.
> > > - The above also causes more serious problem for example for thinly
> > >   provisioned storage (sparse images backed storage as well), because
> > >   instead of reusing blocks which are already provisioned we would try
> > >   to use new blocks. This would unnecessarily drain storage free blocks
> > >   pool.
> > > - This will also cause blocks to be allocated further from the given
> > >   goal than it's necessary. Consider for example truncating, or removing
> > >   and rewriting the file in the loop. This workload will never reuse
> > >   freed blocks until we continually claim and free all the block in the
> > >   file system.
> > >
> > > Note that file systems like xfs, ext3, or btrfs does not have this
> > > problem. This is simply caused by the notion of global pool.
> > >
> > > Fix this by changing the global goal to be goal per inode. This will
> > > allow us to invalidate the goal every time the inode has been
> > > truncated, or newly created, so in those cases we would try to use
> > > the proper more specific goal which is based on inode position.
> >   In principle the patch looks fine to me. Just minor style nit below:
> >
> > >
> > > Signed-off-by: Lukas Czerner <lczerner@redhat.com>
> > > ---
> > >  fs/ext4/ext4.h    |  7 ++++---
> > >  fs/ext4/inode.c   |  8 ++++++++
> > >  fs/ext4/mballoc.c | 20 ++++++++------------
> > >  3 files changed, 20 insertions(+), 15 deletions(-)
> > >
> > > diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h index 6ed348d..4dffa92
> > > 100644
> > > --- a/fs/ext4/ext4.h
> > > +++ b/fs/ext4/ext4.h
> > > @@ -917,6 +917,10 @@ struct ext4_inode_info {
> > >
> > >  	/* Precomputed uuid+inum+igen checksum for seeding inode
> checksums */
> > >  	__u32 i_csum_seed;
> > > +
> > > +	/* where last allocation was done - for stream allocation */
> > > +	unsigned long i_last_group;
> > > +	unsigned long i_last_start;
> >   Can we use proper types here please? I think they should be
> > ext4_grpblk_t and ext4_group_t. And maybe we should define something
> > like EXT4_INVAL_GRPNO and use that instead of UINT_MAX.
> 
> Sure, it'll look better with named types and defined invalidate value. I'll
> resend the patch soon.
> 
> Thanks!
> -Lukas
> 
> >
> > 								Honza
> > >  };
> > >
> > >  /*
> > > @@ -1242,9 +1246,6 @@ struct ext4_sb_info {
> > >  	unsigned int s_mb_order2_reqs;
> > >  	unsigned int s_mb_group_prealloc;
> > >  	unsigned int s_max_dir_size_kb;
> > > -	/* where last allocation was done - for stream allocation */
> > > -	unsigned long s_mb_last_group;
> > > -	unsigned long s_mb_last_start;
> > >
> > >  	/* stats for buddy allocator */
> > >  	atomic_t s_bal_reqs;	/* number of reqs with len > 1 */
> > > diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c index
> > > 0188e65..07d0434 100644
> > > --- a/fs/ext4/inode.c
> > > +++ b/fs/ext4/inode.c
> > > @@ -3702,6 +3702,10 @@ void ext4_truncate(struct inode *inode)
> > >  	else
> > >  		ext4_ind_truncate(handle, inode);
> > >
> > > +	/* Invalidate last allocation counters */
> > > +	ei->i_last_group = UINT_MAX;
> > > +	ei->i_last_start = UINT_MAX;
> > > +
> >   And
> > >  	up_write(&ei->i_data_sem);
> > >
> > >  	if (IS_SYNC(inode))
> > > @@ -4060,6 +4064,10 @@ struct inode *ext4_iget(struct super_block
> *sb, unsigned long ino)
> > >  	inode->i_generation = le32_to_cpu(raw_inode->i_generation);
> > >  	ei->i_block_group = iloc.block_group;
> > >  	ei->i_last_alloc_group = ~0;
> > > +
> > > +	/* Invalidate last allocation counters */
> > > +	ei->i_last_group = UINT_MAX;
> > > +	ei->i_last_start = UINT_MAX;
> > >  	/*
> > >  	 * NOTE! The in-memory inode i_data array is in little-endian order
> > >  	 * even on big-endian machines: we do NOT byteswap the block
> numbers!
> > > diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c index
> > > a9ff5e5..6c23666 100644
> > > --- a/fs/ext4/mballoc.c
> > > +++ b/fs/ext4/mballoc.c
> > > @@ -1591,7 +1591,6 @@ static int mb_mark_used(struct ext4_buddy
> > > *e4b, struct ext4_free_extent *ex)  static void
> ext4_mb_use_best_found(struct ext4_allocation_context *ac,
> > >  					struct ext4_buddy *e4b)
> > >  {
> > > -	struct ext4_sb_info *sbi = EXT4_SB(ac->ac_sb);
> > >  	int ret;
> > >
> > >  	BUG_ON(ac->ac_b_ex.fe_group != e4b->bd_group); @@ -1622,10
> +1621,8
> > > @@ static void ext4_mb_use_best_found(struct
> ext4_allocation_context *ac,
> > >  	get_page(ac->ac_buddy_page);
> > >  	/* store last allocated for subsequent stream allocation */
> > >  	if (ac->ac_flags & EXT4_MB_STREAM_ALLOC) {
> > > -		spin_lock(&sbi->s_md_lock);
> > > -		sbi->s_mb_last_group = ac->ac_f_ex.fe_group;
> > > -		sbi->s_mb_last_start = ac->ac_f_ex.fe_start;
> > > -		spin_unlock(&sbi->s_md_lock);
> > > +		EXT4_I(ac->ac_inode)->i_last_group = ac-
> >ac_f_ex.fe_group;
> > > +		EXT4_I(ac->ac_inode)->i_last_start = ac->ac_f_ex.fe_start;
> > >  	}
> > >  }
> > >
> > > @@ -2080,13 +2077,12 @@ ext4_mb_regular_allocator(struct
> ext4_allocation_context *ac)
> > >  			ac->ac_2order = i - 1;
> > >  	}
> > >
> > > -	/* if stream allocation is enabled, use global goal */
> > > -	if (ac->ac_flags & EXT4_MB_STREAM_ALLOC) {
> > > -		/* TBD: may be hot point */
> > > -		spin_lock(&sbi->s_md_lock);
> > > -		ac->ac_g_ex.fe_group = sbi->s_mb_last_group;
> > > -		ac->ac_g_ex.fe_start = sbi->s_mb_last_start;
> > > -		spin_unlock(&sbi->s_md_lock);
> > > +	/* if stream allocation is enabled and per inode goal is
> > > +	 * set, use it */
> > > +	if ((ac->ac_flags & EXT4_MB_STREAM_ALLOC) &&
> > > +	   (EXT4_I(ac->ac_inode)->i_last_start != UINT_MAX)) {
> > > +		ac->ac_g_ex.fe_group = EXT4_I(ac->ac_inode)-
> >i_last_group;
> > > +		ac->ac_g_ex.fe_start = EXT4_I(ac->ac_inode)->i_last_start;
> > >  	}
> > >
> > >  	/* Let's just scan groups to find more-less suitable blocks */
> > > --
> > > 1.8.3.1
> > >
> > > --
> > > To unsubscribe from this list: send the line "unsubscribe
> > > linux-ext4" in the body of a message to majordomo@vger.kernel.org
> > > More majordomo info at  http://vger.kernel.org/majordomo-info.html
> >
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[PATCH v2] ext4: Try to better reuse recently freed space

[Lukas Czerner]

Currently if the block allocator can not find the goal to allocate we
would use global goal for stream allocation. However the global goal
(s_mb_last_group and s_mb_last_start) will move further every time such
allocation appears and never move backwards.

This causes several problems in certain scenarios:

- the goal will move further and further preventing us from reusing
  space which might have been freed since then. This is ok from the file
  system point of view because we will reuse that space eventually,
  however we're allocating block from slower parts of the spinning disk
  even though it might not be necessary.
- The above also causes more serious problem for example for thinly
  provisioned storage (sparse images backed storage as well), because
  instead of reusing blocks which are already provisioned we would try
  to use new blocks. This would unnecessarily drain storage free blocks
  pool.
- This will also cause blocks to be allocated further from the given
  goal than it's necessary. Consider for example truncating, or removing
  and rewriting the file in the loop. This workload will never reuse
  freed blocks until we continually claim and free all the block in the
  file system.

Note that file systems like xfs, ext3, or btrfs does not have this
problem. This is simply caused by the notion of global pool.

Fix this by changing the global goal to be goal per inode. This will
allow us to invalidate the goal every time the inode has been truncated,
or newly created, so in those cases we would try to use the proper more
specific goal which is based on inode position.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
---
v2: Use proper types. Define invalid value

 fs/ext4/ext4.h    | 12 +++++++++---
 fs/ext4/inode.c   |  8 ++++++++
 fs/ext4/mballoc.c | 20 ++++++++------------
 3 files changed, 25 insertions(+), 15 deletions(-)

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index 6ed348d..d3e9d10 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -917,9 +917,18 @@ struct ext4_inode_info {
 
 	/* Precomputed uuid+inum+igen checksum for seeding inode checksums */
 	__u32 i_csum_seed;
+
+	/* Where last allocation was done - for stream allocation */
+	ext4_group_t i_last_group;
+	ext4_grpblk_t i_last_start;
 };
 
 /*
+ * Invalid value for last allocation pointer
+ */
+#define EXT4_INVAL_GRPNO		UINT_MAX
+
+/*
  * File system states
  */
 #define	EXT4_VALID_FS			0x0001	/* Unmounted cleanly */
@@ -1242,9 +1251,6 @@ struct ext4_sb_info {
 	unsigned int s_mb_order2_reqs;
 	unsigned int s_mb_group_prealloc;
 	unsigned int s_max_dir_size_kb;
-	/* where last allocation was done - for stream allocation */
-	unsigned long s_mb_last_group;
-	unsigned long s_mb_last_start;
 
 	/* stats for buddy allocator */
 	atomic_t s_bal_reqs;	/* number of reqs with len > 1 */
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 0188e65..8d3e67c 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -3702,6 +3702,10 @@ void ext4_truncate(struct inode *inode)
 	else
 		ext4_ind_truncate(handle, inode);
 
+	/* Invalidate last allocation pointers */
+	ei->i_last_group = EXT4_INVAL_GRPNO;
+	ei->i_last_start = 0;
+
 	up_write(&ei->i_data_sem);
 
 	if (IS_SYNC(inode))
@@ -4060,6 +4064,10 @@ struct inode *ext4_iget(struct super_block *sb, unsigned long ino)
 	inode->i_generation = le32_to_cpu(raw_inode->i_generation);
 	ei->i_block_group = iloc.block_group;
 	ei->i_last_alloc_group = ~0;
+
+	/* Invalidate last allocation counters */
+	ei->i_last_group = EXT4_INVAL_GRPNO;
+	ei->i_last_start = 0;
 	/*
 	 * NOTE! The in-memory inode i_data array is in little-endian order
 	 * even on big-endian machines: we do NOT byteswap the block numbers!
diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
index a9ff5e5..7609f77 100644
--- a/fs/ext4/mballoc.c
+++ b/fs/ext4/mballoc.c
@@ -1591,7 +1591,6 @@ static int mb_mark_used(struct ext4_buddy *e4b, struct ext4_free_extent *ex)
 static void ext4_mb_use_best_found(struct ext4_allocation_context *ac,
 					struct ext4_buddy *e4b)
 {
-	struct ext4_sb_info *sbi = EXT4_SB(ac->ac_sb);
 	int ret;
 
 	BUG_ON(ac->ac_b_ex.fe_group != e4b->bd_group);
@@ -1622,10 +1621,8 @@ static void ext4_mb_use_best_found(struct ext4_allocation_context *ac,
 	get_page(ac->ac_buddy_page);
 	/* store last allocated for subsequent stream allocation */
 	if (ac->ac_flags & EXT4_MB_STREAM_ALLOC) {
-		spin_lock(&sbi->s_md_lock);
-		sbi->s_mb_last_group = ac->ac_f_ex.fe_group;
-		sbi->s_mb_last_start = ac->ac_f_ex.fe_start;
-		spin_unlock(&sbi->s_md_lock);
+		EXT4_I(ac->ac_inode)->i_last_group = ac->ac_f_ex.fe_group;
+		EXT4_I(ac->ac_inode)->i_last_start = ac->ac_f_ex.fe_start;
 	}
 }
 
@@ -2080,13 +2077,12 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
 			ac->ac_2order = i - 1;
 	}
 
-	/* if stream allocation is enabled, use global goal */
-	if (ac->ac_flags & EXT4_MB_STREAM_ALLOC) {
-		/* TBD: may be hot point */
-		spin_lock(&sbi->s_md_lock);
-		ac->ac_g_ex.fe_group = sbi->s_mb_last_group;
-		ac->ac_g_ex.fe_start = sbi->s_mb_last_start;
-		spin_unlock(&sbi->s_md_lock);
+	/* if stream allocation is enabled and per inode goal is
+	 * set, use it */
+	if ((ac->ac_flags & EXT4_MB_STREAM_ALLOC) &&
+	   (EXT4_I(ac->ac_inode)->i_last_group != EXT4_INVAL_GRPNO)) {
+		ac->ac_g_ex.fe_group = EXT4_I(ac->ac_inode)->i_last_group;
+		ac->ac_g_ex.fe_start = EXT4_I(ac->ac_inode)->i_last_start;
 	}
 
 	/* Let's just scan groups to find more-less suitable blocks */
-- 
1.8.3.1

Re: [PATCH v2] ext4: Try to better reuse recently freed space

[Jan Kara]

On Mon 08-07-13 09:38:27, Lukas Czerner wrote:
> Currently if the block allocator can not find the goal to allocate we
> would use global goal for stream allocation. However the global goal
> (s_mb_last_group and s_mb_last_start) will move further every time such
> allocation appears and never move backwards.
> 
> This causes several problems in certain scenarios:
> 
> - the goal will move further and further preventing us from reusing
>   space which might have been freed since then. This is ok from the file
>   system point of view because we will reuse that space eventually,
>   however we're allocating block from slower parts of the spinning disk
>   even though it might not be necessary.
> - The above also causes more serious problem for example for thinly
>   provisioned storage (sparse images backed storage as well), because
>   instead of reusing blocks which are already provisioned we would try
>   to use new blocks. This would unnecessarily drain storage free blocks
>   pool.
> - This will also cause blocks to be allocated further from the given
>   goal than it's necessary. Consider for example truncating, or removing
>   and rewriting the file in the loop. This workload will never reuse
>   freed blocks until we continually claim and free all the block in the
>   file system.
> 
> Note that file systems like xfs, ext3, or btrfs does not have this
> problem. This is simply caused by the notion of global pool.
> 
> Fix this by changing the global goal to be goal per inode. This will
> allow us to invalidate the goal every time the inode has been truncated,
> or newly created, so in those cases we would try to use the proper more
> specific goal which is based on inode position.
  When looking at your patch for second time, I started wondering, whether
we need per-inode stream goal at all. We already do set goal in the
allocation request for mballoc (ar->goal) e.g. in ext4_ext_find_goal().
It seems strange to then reset it inside mballoc and I don't even think
mballoc will change it to something else now when the goal is per-inode and
not global.

Which makes me think again about why someone introduced global goal for
stream allocations in the first place? Maybe the motivation was that at
each moment we will have only one area from where we allocate streaming
data (large chunks of blocks) which may presumably reduce fragmentation.
I've added Andreas to CC, maybe he'll remember the purpose.

								Honza

> Signed-off-by: Lukas Czerner <lczerner@redhat.com>
> ---
> v2: Use proper types. Define invalid value
> 
>  fs/ext4/ext4.h    | 12 +++++++++---
>  fs/ext4/inode.c   |  8 ++++++++
>  fs/ext4/mballoc.c | 20 ++++++++------------
>  3 files changed, 25 insertions(+), 15 deletions(-)
> 
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index 6ed348d..d3e9d10 100644
> --- a/fs/ext4/ext4.h
> +++ b/fs/ext4/ext4.h
> @@ -917,9 +917,18 @@ struct ext4_inode_info {
>  
>  	/* Precomputed uuid+inum+igen checksum for seeding inode checksums */
>  	__u32 i_csum_seed;
> +
> +	/* Where last allocation was done - for stream allocation */
> +	ext4_group_t i_last_group;
> +	ext4_grpblk_t i_last_start;
>  };
>  
>  /*
> + * Invalid value for last allocation pointer
> + */
> +#define EXT4_INVAL_GRPNO		UINT_MAX
> +
> +/*
>   * File system states
>   */
>  #define	EXT4_VALID_FS			0x0001	/* Unmounted cleanly */
> @@ -1242,9 +1251,6 @@ struct ext4_sb_info {
>  	unsigned int s_mb_order2_reqs;
>  	unsigned int s_mb_group_prealloc;
>  	unsigned int s_max_dir_size_kb;
> -	/* where last allocation was done - for stream allocation */
> -	unsigned long s_mb_last_group;
> -	unsigned long s_mb_last_start;
>  
>  	/* stats for buddy allocator */
>  	atomic_t s_bal_reqs;	/* number of reqs with len > 1 */
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index 0188e65..8d3e67c 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -3702,6 +3702,10 @@ void ext4_truncate(struct inode *inode)
>  	else
>  		ext4_ind_truncate(handle, inode);
>  
> +	/* Invalidate last allocation pointers */
> +	ei->i_last_group = EXT4_INVAL_GRPNO;
> +	ei->i_last_start = 0;
> +
>  	up_write(&ei->i_data_sem);
>  
>  	if (IS_SYNC(inode))
> @@ -4060,6 +4064,10 @@ struct inode *ext4_iget(struct super_block *sb, unsigned long ino)
>  	inode->i_generation = le32_to_cpu(raw_inode->i_generation);
>  	ei->i_block_group = iloc.block_group;
>  	ei->i_last_alloc_group = ~0;
> +
> +	/* Invalidate last allocation counters */
> +	ei->i_last_group = EXT4_INVAL_GRPNO;
> +	ei->i_last_start = 0;
>  	/*
>  	 * NOTE! The in-memory inode i_data array is in little-endian order
>  	 * even on big-endian machines: we do NOT byteswap the block numbers!
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index a9ff5e5..7609f77 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -1591,7 +1591,6 @@ static int mb_mark_used(struct ext4_buddy *e4b, struct ext4_free_extent *ex)
>  static void ext4_mb_use_best_found(struct ext4_allocation_context *ac,
>  					struct ext4_buddy *e4b)
>  {
> -	struct ext4_sb_info *sbi = EXT4_SB(ac->ac_sb);
>  	int ret;
>  
>  	BUG_ON(ac->ac_b_ex.fe_group != e4b->bd_group);
> @@ -1622,10 +1621,8 @@ static void ext4_mb_use_best_found(struct ext4_allocation_context *ac,
>  	get_page(ac->ac_buddy_page);
>  	/* store last allocated for subsequent stream allocation */
>  	if (ac->ac_flags & EXT4_MB_STREAM_ALLOC) {
> -		spin_lock(&sbi->s_md_lock);
> -		sbi->s_mb_last_group = ac->ac_f_ex.fe_group;
> -		sbi->s_mb_last_start = ac->ac_f_ex.fe_start;
> -		spin_unlock(&sbi->s_md_lock);
> +		EXT4_I(ac->ac_inode)->i_last_group = ac->ac_f_ex.fe_group;
> +		EXT4_I(ac->ac_inode)->i_last_start = ac->ac_f_ex.fe_start;
>  	}
>  }
>  
> @@ -2080,13 +2077,12 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
>  			ac->ac_2order = i - 1;
>  	}
>  
> -	/* if stream allocation is enabled, use global goal */
> -	if (ac->ac_flags & EXT4_MB_STREAM_ALLOC) {
> -		/* TBD: may be hot point */
> -		spin_lock(&sbi->s_md_lock);
> -		ac->ac_g_ex.fe_group = sbi->s_mb_last_group;
> -		ac->ac_g_ex.fe_start = sbi->s_mb_last_start;
> -		spin_unlock(&sbi->s_md_lock);
> +	/* if stream allocation is enabled and per inode goal is
> +	 * set, use it */
> +	if ((ac->ac_flags & EXT4_MB_STREAM_ALLOC) &&
> +	   (EXT4_I(ac->ac_inode)->i_last_group != EXT4_INVAL_GRPNO)) {
> +		ac->ac_g_ex.fe_group = EXT4_I(ac->ac_inode)->i_last_group;
> +		ac->ac_g_ex.fe_start = EXT4_I(ac->ac_inode)->i_last_start;
>  	}
>  
>  	/* Let's just scan groups to find more-less suitable blocks */
> -- 
> 1.8.3.1
> 
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR

Re: [PATCH v2] ext4: Try to better reuse recently freed space

[Lukáš Czerner]

On Mon, 8 Jul 2013, Jan Kara wrote:

> Date: Mon, 8 Jul 2013 10:56:03 +0200
> From: Jan Kara <jack@suse.cz>
> To: Lukas Czerner <lczerner@redhat.com>
> Cc: linux-ext4@vger.kernel.org, jack@suse.cz,
>     Andreas Dilger <adilger.kernel@dilger.ca>
> Subject: Re: [PATCH v2] ext4: Try to better reuse recently freed space
> 
> On Mon 08-07-13 09:38:27, Lukas Czerner wrote:
> > Currently if the block allocator can not find the goal to allocate we
> > would use global goal for stream allocation. However the global goal
> > (s_mb_last_group and s_mb_last_start) will move further every time such
> > allocation appears and never move backwards.
> > 
> > This causes several problems in certain scenarios:
> > 
> > - the goal will move further and further preventing us from reusing
> >   space which might have been freed since then. This is ok from the file
> >   system point of view because we will reuse that space eventually,
> >   however we're allocating block from slower parts of the spinning disk
> >   even though it might not be necessary.
> > - The above also causes more serious problem for example for thinly
> >   provisioned storage (sparse images backed storage as well), because
> >   instead of reusing blocks which are already provisioned we would try
> >   to use new blocks. This would unnecessarily drain storage free blocks
> >   pool.
> > - This will also cause blocks to be allocated further from the given
> >   goal than it's necessary. Consider for example truncating, or removing
> >   and rewriting the file in the loop. This workload will never reuse
> >   freed blocks until we continually claim and free all the block in the
> >   file system.
> > 
> > Note that file systems like xfs, ext3, or btrfs does not have this
> > problem. This is simply caused by the notion of global pool.
> > 
> > Fix this by changing the global goal to be goal per inode. This will
> > allow us to invalidate the goal every time the inode has been truncated,
> > or newly created, so in those cases we would try to use the proper more
> > specific goal which is based on inode position.
>   When looking at your patch for second time, I started wondering, whether
> we need per-inode stream goal at all. We already do set goal in the
> allocation request for mballoc (ar->goal) e.g. in ext4_ext_find_goal().
> It seems strange to then reset it inside mballoc and I don't even think
> mballoc will change it to something else now when the goal is per-inode and
> not global.

Yes, we do set the goal in the allocation request and it is supposed
to be the "best" goal. However sometimes it can not be fulfilled
because we do not have any free block at "goal".

That's when the global (or per-inode) goal comes into play. I suppose
that there was several reasons for that. First of all it makes it
easier for allocator, because it can directly jump at the point
where we allocated last time and it is likely that there is some
free space to allocate from - so the benefit is that we do not have
to walk all the space in between which is likely to be allocated.

The second might be to keep new allocation together.

This is also what is creating the problem which this patch is trying
to fix. We assume that when the allocation request goal can not be
fulfilled then all the space between that goal and the global goal
is allocated so we jump directly to the global goal. However it
might not be the case at all.

With per-inode goal we can jump directly to the point we allocated
last time for given inode and start searching free space from there
in the case that allocation request goal can not be fulfilled for
some reason. Also we're able to invalidate that goal when we
truncate the file so we can start searching from the allocation
request goal.

Obviously the per-inode goal does not have all the benefits of the
global goal, because we're less likely to have free space right
after per-inode goal then after global goal, but it does not have that
nasty side-effect of touching all the block in the file system
unnecessarily, getting more and more distant from the allocation
request goal even though it's not always needed, and unnecessarily
pushing the allocation to the slower parts of the disk in some
cases.

-Lukas

> 
> Which makes me think again about why someone introduced global goal for
> stream allocations in the first place? Maybe the motivation was that at
> each moment we will have only one area from where we allocate streaming
> data (large chunks of blocks) which may presumably reduce fragmentation.
> I've added Andreas to CC, maybe he'll remember the purpose.
> 
> 								Honza
> 
> > Signed-off-by: Lukas Czerner <lczerner@redhat.com>
> > ---
> > v2: Use proper types. Define invalid value
> > 
> >  fs/ext4/ext4.h    | 12 +++++++++---
> >  fs/ext4/inode.c   |  8 ++++++++
> >  fs/ext4/mballoc.c | 20 ++++++++------------
> >  3 files changed, 25 insertions(+), 15 deletions(-)
> > 
> > diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> > index 6ed348d..d3e9d10 100644
> > --- a/fs/ext4/ext4.h
> > +++ b/fs/ext4/ext4.h
> > @@ -917,9 +917,18 @@ struct ext4_inode_info {
> >  
> >  	/* Precomputed uuid+inum+igen checksum for seeding inode checksums */
> >  	__u32 i_csum_seed;
> > +
> > +	/* Where last allocation was done - for stream allocation */
> > +	ext4_group_t i_last_group;
> > +	ext4_grpblk_t i_last_start;
> >  };
> >  
> >  /*
> > + * Invalid value for last allocation pointer
> > + */
> > +#define EXT4_INVAL_GRPNO		UINT_MAX
> > +
> > +/*
> >   * File system states
> >   */
> >  #define	EXT4_VALID_FS			0x0001	/* Unmounted cleanly */
> > @@ -1242,9 +1251,6 @@ struct ext4_sb_info {
> >  	unsigned int s_mb_order2_reqs;
> >  	unsigned int s_mb_group_prealloc;
> >  	unsigned int s_max_dir_size_kb;
> > -	/* where last allocation was done - for stream allocation */
> > -	unsigned long s_mb_last_group;
> > -	unsigned long s_mb_last_start;
> >  
> >  	/* stats for buddy allocator */
> >  	atomic_t s_bal_reqs;	/* number of reqs with len > 1 */
> > diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> > index 0188e65..8d3e67c 100644
> > --- a/fs/ext4/inode.c
> > +++ b/fs/ext4/inode.c
> > @@ -3702,6 +3702,10 @@ void ext4_truncate(struct inode *inode)
> >  	else
> >  		ext4_ind_truncate(handle, inode);
> >  
> > +	/* Invalidate last allocation pointers */
> > +	ei->i_last_group = EXT4_INVAL_GRPNO;
> > +	ei->i_last_start = 0;
> > +
> >  	up_write(&ei->i_data_sem);
> >  
> >  	if (IS_SYNC(inode))
> > @@ -4060,6 +4064,10 @@ struct inode *ext4_iget(struct super_block *sb, unsigned long ino)
> >  	inode->i_generation = le32_to_cpu(raw_inode->i_generation);
> >  	ei->i_block_group = iloc.block_group;
> >  	ei->i_last_alloc_group = ~0;
> > +
> > +	/* Invalidate last allocation counters */
> > +	ei->i_last_group = EXT4_INVAL_GRPNO;
> > +	ei->i_last_start = 0;
> >  	/*
> >  	 * NOTE! The in-memory inode i_data array is in little-endian order
> >  	 * even on big-endian machines: we do NOT byteswap the block numbers!
> > diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> > index a9ff5e5..7609f77 100644
> > --- a/fs/ext4/mballoc.c
> > +++ b/fs/ext4/mballoc.c
> > @@ -1591,7 +1591,6 @@ static int mb_mark_used(struct ext4_buddy *e4b, struct ext4_free_extent *ex)
> >  static void ext4_mb_use_best_found(struct ext4_allocation_context *ac,
> >  					struct ext4_buddy *e4b)
> >  {
> > -	struct ext4_sb_info *sbi = EXT4_SB(ac->ac_sb);
> >  	int ret;
> >  
> >  	BUG_ON(ac->ac_b_ex.fe_group != e4b->bd_group);
> > @@ -1622,10 +1621,8 @@ static void ext4_mb_use_best_found(struct ext4_allocation_context *ac,
> >  	get_page(ac->ac_buddy_page);
> >  	/* store last allocated for subsequent stream allocation */
> >  	if (ac->ac_flags & EXT4_MB_STREAM_ALLOC) {
> > -		spin_lock(&sbi->s_md_lock);
> > -		sbi->s_mb_last_group = ac->ac_f_ex.fe_group;
> > -		sbi->s_mb_last_start = ac->ac_f_ex.fe_start;
> > -		spin_unlock(&sbi->s_md_lock);
> > +		EXT4_I(ac->ac_inode)->i_last_group = ac->ac_f_ex.fe_group;
> > +		EXT4_I(ac->ac_inode)->i_last_start = ac->ac_f_ex.fe_start;
> >  	}
> >  }
> >  
> > @@ -2080,13 +2077,12 @@ ext4_mb_regular_allocator(struct ext4_allocation_context *ac)
> >  			ac->ac_2order = i - 1;
> >  	}
> >  
> > -	/* if stream allocation is enabled, use global goal */
> > -	if (ac->ac_flags & EXT4_MB_STREAM_ALLOC) {
> > -		/* TBD: may be hot point */
> > -		spin_lock(&sbi->s_md_lock);
> > -		ac->ac_g_ex.fe_group = sbi->s_mb_last_group;
> > -		ac->ac_g_ex.fe_start = sbi->s_mb_last_start;
> > -		spin_unlock(&sbi->s_md_lock);
> > +	/* if stream allocation is enabled and per inode goal is
> > +	 * set, use it */
> > +	if ((ac->ac_flags & EXT4_MB_STREAM_ALLOC) &&
> > +	   (EXT4_I(ac->ac_inode)->i_last_group != EXT4_INVAL_GRPNO)) {
> > +		ac->ac_g_ex.fe_group = EXT4_I(ac->ac_inode)->i_last_group;
> > +		ac->ac_g_ex.fe_start = EXT4_I(ac->ac_inode)->i_last_start;
> >  	}
> >  
> >  	/* Let's just scan groups to find more-less suitable blocks */
> > -- 
> > 1.8.3.1
> > 
> 

Re: [PATCH v2] ext4: Try to better reuse recently freed space

[Jan Kara]

On Mon 08-07-13 11:24:01, Lukáš Czerner wrote:
> On Mon, 8 Jul 2013, Jan Kara wrote:
> 
> > Date: Mon, 8 Jul 2013 10:56:03 +0200
> > From: Jan Kara <jack@suse.cz>
> > To: Lukas Czerner <lczerner@redhat.com>
> > Cc: linux-ext4@vger.kernel.org, jack@suse.cz,
> >     Andreas Dilger <adilger.kernel@dilger.ca>
> > Subject: Re: [PATCH v2] ext4: Try to better reuse recently freed space
> > 
> > On Mon 08-07-13 09:38:27, Lukas Czerner wrote:
> > > Currently if the block allocator can not find the goal to allocate we
> > > would use global goal for stream allocation. However the global goal
> > > (s_mb_last_group and s_mb_last_start) will move further every time such
> > > allocation appears and never move backwards.
> > > 
> > > This causes several problems in certain scenarios:
> > > 
> > > - the goal will move further and further preventing us from reusing
> > >   space which might have been freed since then. This is ok from the file
> > >   system point of view because we will reuse that space eventually,
> > >   however we're allocating block from slower parts of the spinning disk
> > >   even though it might not be necessary.
> > > - The above also causes more serious problem for example for thinly
> > >   provisioned storage (sparse images backed storage as well), because
> > >   instead of reusing blocks which are already provisioned we would try
> > >   to use new blocks. This would unnecessarily drain storage free blocks
> > >   pool.
> > > - This will also cause blocks to be allocated further from the given
> > >   goal than it's necessary. Consider for example truncating, or removing
> > >   and rewriting the file in the loop. This workload will never reuse
> > >   freed blocks until we continually claim and free all the block in the
> > >   file system.
> > > 
> > > Note that file systems like xfs, ext3, or btrfs does not have this
> > > problem. This is simply caused by the notion of global pool.
> > > 
> > > Fix this by changing the global goal to be goal per inode. This will
> > > allow us to invalidate the goal every time the inode has been truncated,
> > > or newly created, so in those cases we would try to use the proper more
> > > specific goal which is based on inode position.
> >   When looking at your patch for second time, I started wondering, whether
> > we need per-inode stream goal at all. We already do set goal in the
> > allocation request for mballoc (ar->goal) e.g. in ext4_ext_find_goal().
> > It seems strange to then reset it inside mballoc and I don't even think
> > mballoc will change it to something else now when the goal is per-inode and
> > not global.
> 
> Yes, we do set the goal in the allocation request and it is supposed
> to be the "best" goal. However sometimes it can not be fulfilled
> because we do not have any free block at "goal".
> 
> That's when the global (or per-inode) goal comes into play. I suppose
> that there was several reasons for that. First of all it makes it
> easier for allocator, because it can directly jump at the point
> where we allocated last time and it is likely that there is some
> free space to allocate from - so the benefit is that we do not have
> to walk all the space in between which is likely to be allocated.
  Yep, but my question is: If we have per-inode streaming goal, can you
show an example when the "best" goal will be different from the "streaming"
goal? Because from a (I admit rather quick) look at how each of these is
computed, it seems that both will point after the next allocated block in
case of streaming IO.

								Honza
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR
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Re: [PATCH v2] ext4: Try to better reuse recently freed space

[Andreas Dilger]

On 2013-07-08, at 5:59 AM, Jan Kara wrote:
> On Mon 08-07-13 11:24:01, Lukáš Czerner wrote:
>> On Mon, 8 Jul 2013, Jan Kara wrote:
>>> On Mon, 8 Jul 2013, Jan Kara <jack@suse.cz> wrote:
>>> On Mon 08-07-13 09:38:27, Lukas Czerner wrote:
>>>> Currently if the block allocator can not find the goal to
>>>> allocate we would use global goal for stream allocation.
>>>> However the global goal (s_mb_last_group and s_mb_last_start) will move further every time such allocation appears and
>>>> never move backwards.
>>>> 
>>>> This causes several problems in certain scenarios:
>>>> - the goal will move further and further preventing us from
>>>>   reusing space which might have been freed since then. This
>>>>   is ok from the file system point of view because we will
>>>>   reuse that space eventually, however we're allocating block
>>>>   from slower parts of the spinning disk even though it might
>>>>   not be necessary.
>>>> - The above also causes more serious problem for example for
>>>>   thinly  provisioned storage (sparse images backed storage
>>>>   as well), because instead of reusing blocks which are
>>>>   already provisioned we would try to use new blocks. This
>>>>   would unnecessarily drain storage free blocks pool.
>>>> - This will also cause blocks to be allocated further from
>>>>   the given goal than it's necessary. Consider for example
>>>>   truncating, or removing and rewriting the file in the loop.
>>>> This workload will never reuse freed blocks until we continually
>>>> claim and free all the block in the file system.
>>>> 
>>>> Note that file systems like xfs, ext3, or btrfs does not have this problem. This is simply caused by the notion of global pool.
>>>> 
>>>> Fix this by changing the global goal to be goal per inode. This will allow us to invalidate the goal every time the inode has
>>>> been truncated, or newly created, so in those cases we would try
>>>> to use the proper more specific goal which is based on inode
>>>> position.
>>> When looking at your patch for second time, I started wondering,
>>> whether we need per-inode stream goal at all. We already do set
>>> goal in the allocation request for mballoc (ar->goal) e.g. in
>>> ext4_ext_find_goal().
>>> It seems strange to then reset it inside mballoc and I don't
>>> even think mballoc will change it to something else now when
>>> the goal is per-inode and not global.
>> 
>> Yes, we do set the goal in the allocation request and it is supposed
>> to be the "best" goal. However sometimes it can not be fulfilled
>> because we do not have any free block at "goal".
>> 
>> That's when the global (or per-inode) goal comes into play. I suppose
>> that there was several reasons for that. First of all it makes it
>> easier for allocator, because it can directly jump at the point
>> where we allocated last time and it is likely that there is some
>> free space to allocate from - so the benefit is that we do not have
>> to walk all the space in between which is likely to be allocated.
> 
> Yep, but my question is: If we have per-inode streaming goal, can
> you show an example when the "best" goal will be different from the
> "streaming" goal? Because from a (I admit rather quick) look at how
> each of these is computed, it seems that both will point after the
> next allocated block in case of streaming IO.

There were a few goals with the design of the mballoc allocator:
- keep large allocations sized and aligned on RAID chunks
- pack small allocations together, so they can fit into a
  single RAID chunk and avoid many read-modify-write cycles
- keep allocations relatively close together, so that seeking is
  minimized and it doesn't search through fragmented free space

It was designed for high-bandwidth streaming IO, and not small random IO as is seen with VM images and thin-provisioned storage.  That said, with TRIM and sparse storage, there is no guarantee that offset within the virtual device has any association with the real disk, so I'm not sure it makes sense to optimize for this case.

The ext4 code will already bias new allocations towards the beginning of the disk because this is how inodes are allocated and this influences each inode's block allocation.  The global target allows the large allocations to get out of the mess of fragmented free space and into other parts of the filesystem that are not as heavily used.

Rather than tweaking the global target (which is currently just a simple round-robin), it probably makes sense to look at a global extent map for free space, so that it is easier to find large chunks of free space.  It is still desirable to keep these allocations closer together, otherwise the "optimum" location for several large allocations may be on different ends of the disk and the situation would be far worse than making periodic writes over the whole disk.

Cheers, Andreas





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Re: [PATCH v2] ext4: Try to better reuse recently freed space

[Lukáš Czerner]

[-- Attachment #1: Type: TEXT/PLAIN, Size: 4760 bytes --]

On Mon, 8 Jul 2013, Jan Kara wrote:

> Date: Mon, 8 Jul 2013 13:59:51 +0200
> From: Jan Kara <jack@suse.cz>
> To: Lukáš Czerner <lczerner@redhat.com>
> Cc: Jan Kara <jack@suse.cz>, linux-ext4@vger.kernel.org,
>     Andreas Dilger <adilger.kernel@dilger.ca>
> Subject: Re: [PATCH v2] ext4: Try to better reuse recently freed space
> 
> On Mon 08-07-13 11:24:01, Lukáš Czerner wrote:
> > On Mon, 8 Jul 2013, Jan Kara wrote:
> > 
> > > Date: Mon, 8 Jul 2013 10:56:03 +0200
> > > From: Jan Kara <jack@suse.cz>
> > > To: Lukas Czerner <lczerner@redhat.com>
> > > Cc: linux-ext4@vger.kernel.org, jack@suse.cz,
> > >     Andreas Dilger <adilger.kernel@dilger.ca>
> > > Subject: Re: [PATCH v2] ext4: Try to better reuse recently freed space
> > > 
> > > On Mon 08-07-13 09:38:27, Lukas Czerner wrote:
> > > > Currently if the block allocator can not find the goal to allocate we
> > > > would use global goal for stream allocation. However the global goal
> > > > (s_mb_last_group and s_mb_last_start) will move further every time such
> > > > allocation appears and never move backwards.
> > > > 
> > > > This causes several problems in certain scenarios:
> > > > 
> > > > - the goal will move further and further preventing us from reusing
> > > >   space which might have been freed since then. This is ok from the file
> > > >   system point of view because we will reuse that space eventually,
> > > >   however we're allocating block from slower parts of the spinning disk
> > > >   even though it might not be necessary.
> > > > - The above also causes more serious problem for example for thinly
> > > >   provisioned storage (sparse images backed storage as well), because
> > > >   instead of reusing blocks which are already provisioned we would try
> > > >   to use new blocks. This would unnecessarily drain storage free blocks
> > > >   pool.
> > > > - This will also cause blocks to be allocated further from the given
> > > >   goal than it's necessary. Consider for example truncating, or removing
> > > >   and rewriting the file in the loop. This workload will never reuse
> > > >   freed blocks until we continually claim and free all the block in the
> > > >   file system.
> > > > 
> > > > Note that file systems like xfs, ext3, or btrfs does not have this
> > > > problem. This is simply caused by the notion of global pool.
> > > > 
> > > > Fix this by changing the global goal to be goal per inode. This will
> > > > allow us to invalidate the goal every time the inode has been truncated,
> > > > or newly created, so in those cases we would try to use the proper more
> > > > specific goal which is based on inode position.
> > >   When looking at your patch for second time, I started wondering, whether
> > > we need per-inode stream goal at all. We already do set goal in the
> > > allocation request for mballoc (ar->goal) e.g. in ext4_ext_find_goal().
> > > It seems strange to then reset it inside mballoc and I don't even think
> > > mballoc will change it to something else now when the goal is per-inode and
> > > not global.
> > 
> > Yes, we do set the goal in the allocation request and it is supposed
> > to be the "best" goal. However sometimes it can not be fulfilled
> > because we do not have any free block at "goal".
> > 
> > That's when the global (or per-inode) goal comes into play. I suppose
> > that there was several reasons for that. First of all it makes it
> > easier for allocator, because it can directly jump at the point
> > where we allocated last time and it is likely that there is some
> > free space to allocate from - so the benefit is that we do not have
> > to walk all the space in between which is likely to be allocated.
>   Yep, but my question is: If we have per-inode streaming goal, can you
> show an example when the "best" goal will be different from the "streaming"
> goal? Because from a (I admit rather quick) look at how each of these is
> computed, it seems that both will point after the next allocated block in
> case of streaming IO.

EXT4_MB_STREAM_ALLOC or "streaming IO" is quite misleading name for
what we have in ext4. It simply means that the file (or allocation)
is bigger than certain threshold.

So I think that one example would be when writing in the middle of
sparse file when other processes might have already allocated
requested blocks. This might be the case for file system images for
example. Also for some reason I am seeing this when writing into
file system image even though there are no other processes
allocating from that file system.

Simply hacking ext4 to print out the numbers when the goals differs
and running xfstests shows that there are cases where it differs and
where it helps to allocate from the per-inode goal.

-Lukas

> 
> 								Honza
> 

Re: [PATCH v2] ext4: Try to better reuse recently freed space

[Lukáš Czerner]

[-- Attachment #1: Type: TEXT/PLAIN, Size: 6399 bytes --]

On Mon, 8 Jul 2013, Andreas Dilger wrote:

> Date: Mon, 8 Jul 2013 15:27:21 -0600
> From: Andreas Dilger <adilger@dilger.ca>
> To: Jan Kara <jack@suse.cz>
> Cc: Lukáš Czerner <lczerner@redhat.com>,
>     Alex Zhuravlev <alexey.zhuravlev@intel.com>,
>     "linux-ext4@vger.kernel.org List" <linux-ext4@vger.kernel.org>
> Subject: Re: [PATCH v2] ext4: Try to better reuse recently freed space
> 
> On 2013-07-08, at 5:59 AM, Jan Kara wrote:
> > On Mon 08-07-13 11:24:01, Lukáš Czerner wrote:
> >> On Mon, 8 Jul 2013, Jan Kara wrote:
> >>> On Mon, 8 Jul 2013, Jan Kara <jack@suse.cz> wrote:
> >>> On Mon 08-07-13 09:38:27, Lukas Czerner wrote:
> >>>> Currently if the block allocator can not find the goal to
> >>>> allocate we would use global goal for stream allocation.
> >>>> However the global goal (s_mb_last_group and s_mb_last_start) will move further every time such allocation appears and
> >>>> never move backwards.
> >>>> 
> >>>> This causes several problems in certain scenarios:
> >>>> - the goal will move further and further preventing us from
> >>>>   reusing space which might have been freed since then. This
> >>>>   is ok from the file system point of view because we will
> >>>>   reuse that space eventually, however we're allocating block
> >>>>   from slower parts of the spinning disk even though it might
> >>>>   not be necessary.
> >>>> - The above also causes more serious problem for example for
> >>>>   thinly  provisioned storage (sparse images backed storage
> >>>>   as well), because instead of reusing blocks which are
> >>>>   already provisioned we would try to use new blocks. This
> >>>>   would unnecessarily drain storage free blocks pool.
> >>>> - This will also cause blocks to be allocated further from
> >>>>   the given goal than it's necessary. Consider for example
> >>>>   truncating, or removing and rewriting the file in the loop.
> >>>> This workload will never reuse freed blocks until we continually
> >>>> claim and free all the block in the file system.
> >>>> 
> >>>> Note that file systems like xfs, ext3, or btrfs does not have this problem. This is simply caused by the notion of global pool.
> >>>> 
> >>>> Fix this by changing the global goal to be goal per inode. This will allow us to invalidate the goal every time the inode has
> >>>> been truncated, or newly created, so in those cases we would try
> >>>> to use the proper more specific goal which is based on inode
> >>>> position.
> >>> When looking at your patch for second time, I started wondering,
> >>> whether we need per-inode stream goal at all. We already do set
> >>> goal in the allocation request for mballoc (ar->goal) e.g. in
> >>> ext4_ext_find_goal().
> >>> It seems strange to then reset it inside mballoc and I don't
> >>> even think mballoc will change it to something else now when
> >>> the goal is per-inode and not global.
> >> 
> >> Yes, we do set the goal in the allocation request and it is supposed
> >> to be the "best" goal. However sometimes it can not be fulfilled
> >> because we do not have any free block at "goal".
> >> 
> >> That's when the global (or per-inode) goal comes into play. I suppose
> >> that there was several reasons for that. First of all it makes it
> >> easier for allocator, because it can directly jump at the point
> >> where we allocated last time and it is likely that there is some
> >> free space to allocate from - so the benefit is that we do not have
> >> to walk all the space in between which is likely to be allocated.
> > 
> > Yep, but my question is: If we have per-inode streaming goal, can
> > you show an example when the "best" goal will be different from the
> > "streaming" goal? Because from a (I admit rather quick) look at how
> > each of these is computed, it seems that both will point after the
> > next allocated block in case of streaming IO.
> 
> There were a few goals with the design of the mballoc allocator:
> - keep large allocations sized and aligned on RAID chunks
> - pack small allocations together, so they can fit into a
>   single RAID chunk and avoid many read-modify-write cycles
> - keep allocations relatively close together, so that seeking is
>   minimized and it doesn't search through fragmented free space
> 
> It was designed for high-bandwidth streaming IO, and not small random IO as is seen with VM images and thin-provisioned storage.  That said, with TRIM and sparse storage, there is no guarantee that offset within the virtual device has any association with the real disk, so I'm not sure it makes sense to optimize for this case.

However it is used for small random IO as well. We're going to use
EXT4_MB_STREAM_ALLOC every time when the file, or allocation is
bigger than sbi->s_mb_stream_request. Are you saying that it was not
designed that way ?

Moreover we do not really care about the association of the offset
within the virtual device and the real disk. That's not the problem.
The offset might only be a problem on real disk, where it might be
getting slower as we move further into the file system, but that's
also questionable in some cases.

However with thin-provisioning this might really be a problem
because we would "provision" much more blocks that the file system
really needs, because we're not reusing blocks which has been
recently freed, but rather touching new block. This will
unnecessarily drain thin-provisioning free space pool.

> 
> The ext4 code will already bias new allocations towards the beginning of the disk because this is how inodes are allocated and this influences each inode's block allocation.  The global target allows the large allocations to get out of the mess of fragmented free space and into other parts of the filesystem that are not as heavily used.
> 
> Rather than tweaking the global target (which is currently just a simple round-robin), it probably makes sense to look at a global extent map for free space, so that it is easier to find large chunks of free space.  It is still desirable to keep these allocations closer together, otherwise the "optimum" location for several large allocations may be on different ends of the disk and the situation would be far worse than making periodic writes over the whole disk.

I agre that having a global extent map for free space will be
useful. But this is a different problem even though somewhat
related.

Thanks!
-Lukas

> 
> Cheers, Andreas
> 
> 
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