[PATCH] btrfs: round down stripe size and chunk size to pow of 2

[PATCH] btrfs: round down stripe size and chunk size to pow of 2

[Wang Yugui]

In decide_stripe_size_regular(), when new disk is added to RAID0/RAID10/RAID56,
it is better to alloc/reuse the free space if stripe size is kept or
changed to 1/2. so stripe size and chunk size of pow of 2 will be more
friendly.

Signed-off-by: Wang Yugui <wangyugui@e16-tech.com>
---
 fs/btrfs/volumes.c | 20 +++++++++-----------
 1 file changed, 9 insertions(+), 11 deletions(-)

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 6595755..fab9765 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -5083,9 +5083,9 @@ static void init_alloc_chunk_ctl_policy_regular(
 	if (ctl->type & BTRFS_BLOCK_GROUP_SYSTEM)
 		ctl->devs_max = min_t(int, ctl->devs_max, BTRFS_MAX_DEVS_SYS_CHUNK);
 
-	/* We don't want a chunk larger than 10% of writable space */
-	ctl->max_chunk_size = min(div_factor(fs_devices->total_rw_bytes, 1),
-				  ctl->max_chunk_size);
+	/* We don't want a chunk larger than 1/8 of writable space */
+	ctl->max_chunk_size = min_t(u64, ctl->max_chunk_size,
+			rounddown_pow_of_two(fs_devices->total_rw_bytes >> 3));
 	ctl->dev_extent_min = BTRFS_STRIPE_LEN * ctl->dev_stripes;
 }
 
@@ -5143,10 +5143,9 @@ static void init_alloc_chunk_ctl_policy_zoned(
 		BUG();
 	}
 
-	/* We don't want a chunk larger than 10% of writable space */
-	limit = max(round_down(div_factor(fs_devices->total_rw_bytes, 1),
-			       zone_size),
-		    min_chunk_size);
+	/* We don't want a chunk larger than 1/8 of writable space */
+	limit = max_t(u64, min_chunk_size,
+		rounddown_pow_of_two(fs_devices->total_rw_bytes >> 3));
 	ctl->max_chunk_size = min(limit, ctl->max_chunk_size);
 	ctl->dev_extent_min = zone_size * ctl->dev_stripes;
 }
@@ -5284,13 +5283,12 @@ static int decide_stripe_size_regular(struct alloc_chunk_ctl *ctl,
 	 */
 	if (ctl->stripe_size * data_stripes > ctl->max_chunk_size) {
 		/*
-		 * Reduce stripe_size, round it up to a 16MB boundary again and
+		 * Reduce stripe_size, round down to pow of 2 boundary again and
 		 * then use it, unless it ends up being even bigger than the
 		 * previous value we had already.
 		 */
-		ctl->stripe_size = min(round_up(div_u64(ctl->max_chunk_size,
-							data_stripes), SZ_16M),
-				       ctl->stripe_size);
+		ctl->stripe_size = min_t(u64, ctl->stripe_size,
+			rounddown_pow_of_two(div_u64(ctl->max_chunk_size, data_stripes)));
 	}
 
 	/* Align to BTRFS_STRIPE_LEN */
-- 
2.36.2

Re: [PATCH] btrfs: round down stripe size and chunk size to pow of 2

[Qu Wenruo]

On 2022/8/16 09:46, Wang Yugui wrote:
> In decide_stripe_size_regular(), when new disk is added to RAID0/RAID10/RAID56,
> it is better to alloc/reuse the free space if stripe size is kept or
> changed to 1/2. so stripe size and chunk size of pow of 2 will be more
> friendly.
>
> Signed-off-by: Wang Yugui <wangyugui@e16-tech.com>
> ---
>   fs/btrfs/volumes.c | 20 +++++++++-----------
>   1 file changed, 9 insertions(+), 11 deletions(-)
>
> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> index 6595755..fab9765 100644
> --- a/fs/btrfs/volumes.c
> +++ b/fs/btrfs/volumes.c
> @@ -5083,9 +5083,9 @@ static void init_alloc_chunk_ctl_policy_regular(
>   	if (ctl->type & BTRFS_BLOCK_GROUP_SYSTEM)
>   		ctl->devs_max = min_t(int, ctl->devs_max, BTRFS_MAX_DEVS_SYS_CHUNK);
>
> -	/* We don't want a chunk larger than 10% of writable space */
> -	ctl->max_chunk_size = min(div_factor(fs_devices->total_rw_bytes, 1),
> -				  ctl->max_chunk_size);
> +	/* We don't want a chunk larger than 1/8 of writable space */

For the 1/8 change I'm completely fine.

> +	ctl->max_chunk_size = min_t(u64, ctl->max_chunk_size,
> +			rounddown_pow_of_two(fs_devices->total_rw_bytes >> 3));

But I'm not sure if there is any benefit for the extra
dounwdown_pow_of_two().

Our chunk size doesn't really need to be power of 2 at all.

Any extra explanation on why power of 2 chunk size has any benefit?

>   	ctl->dev_extent_min = BTRFS_STRIPE_LEN * ctl->dev_stripes;
>   }
>
> @@ -5143,10 +5143,9 @@ static void init_alloc_chunk_ctl_policy_zoned(
>   		BUG();
>   	}
>
> -	/* We don't want a chunk larger than 10% of writable space */
> -	limit = max(round_down(div_factor(fs_devices->total_rw_bytes, 1),
> -			       zone_size),
> -		    min_chunk_size);
> +	/* We don't want a chunk larger than 1/8 of writable space */
> +	limit = max_t(u64, min_chunk_size,
> +		rounddown_pow_of_two(fs_devices->total_rw_bytes >> 3));
>   	ctl->max_chunk_size = min(limit, ctl->max_chunk_size);
>   	ctl->dev_extent_min = zone_size * ctl->dev_stripes;
>   }
> @@ -5284,13 +5283,12 @@ static int decide_stripe_size_regular(struct alloc_chunk_ctl *ctl,
>   	 */
>   	if (ctl->stripe_size * data_stripes > ctl->max_chunk_size) {
>   		/*
> -		 * Reduce stripe_size, round it up to a 16MB boundary again and
> +		 * Reduce stripe_size, round down to pow of 2 boundary again and
>   		 * then use it, unless it ends up being even bigger than the
>   		 * previous value we had already.
>   		 */
> -		ctl->stripe_size = min(round_up(div_u64(ctl->max_chunk_size,
> -							data_stripes), SZ_16M),
> -				       ctl->stripe_size);
> +		ctl->stripe_size = min_t(u64, ctl->stripe_size,
> +			rounddown_pow_of_two(div_u64(ctl->max_chunk_size, data_stripes)));

I think this can even be problematic since now stripe_size can be much
smaller than what we want.

Thanks,
Qu

>   	}
>
>   	/* Align to BTRFS_STRIPE_LEN */

Re: [PATCH] btrfs: round down stripe size and chunk size to pow of 2

[Wang Yugui]

Hi,

> On 2022/8/16 09:46, Wang Yugui wrote:
> > In decide_stripe_size_regular(), when new disk is added to RAID0/RAID10/RAID56,
> > it is better to alloc/reuse the free space if stripe size is kept or
> > changed to 1/2. so stripe size and chunk size of pow of 2 will be more
> > friendly.
> >
> > Signed-off-by: Wang Yugui <wangyugui@e16-tech.com>
> > ---
> >   fs/btrfs/volumes.c | 20 +++++++++-----------
> >   1 file changed, 9 insertions(+), 11 deletions(-)
> >
> > diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> > index 6595755..fab9765 100644
> > --- a/fs/btrfs/volumes.c
> > +++ b/fs/btrfs/volumes.c
> > @@ -5083,9 +5083,9 @@ static void init_alloc_chunk_ctl_policy_regular(
> >   	if (ctl->type & BTRFS_BLOCK_GROUP_SYSTEM)
> >   		ctl->devs_max = min_t(int, ctl->devs_max, BTRFS_MAX_DEVS_SYS_CHUNK);
> >
> > -	/* We don't want a chunk larger than 10% of writable space */
> > -	ctl->max_chunk_size = min(div_factor(fs_devices->total_rw_bytes, 1),
> > -				  ctl->max_chunk_size);
> > +	/* We don't want a chunk larger than 1/8 of writable space */
> 
> For the 1/8 change I'm completely fine.
> 
> > +	ctl->max_chunk_size = min_t(u64, ctl->max_chunk_size,
> > +			rounddown_pow_of_two(fs_devices->total_rw_bytes >> 3));
> 
> But I'm not sure if there is any benefit for the extra
> dounwdown_pow_of_two().
> 
> Our chunk size doesn't really need to be power of 2 at all.
> 
> Any extra explanation on why power of 2 chunk size has any benefit?

Becasue stripe_size is roundup_pow_of_two()/rounddown_pow_of_two() later,
so real chunk size is roundup_pow_of_two() * N /rounddown_pow_of_two()
* N.

Here we get max_chunk_size base on  pow of 2 too, so that in
single/RAID1 profile,  max_chunk_size and chunk size will be same.

> >   	ctl->dev_extent_min = BTRFS_STRIPE_LEN * ctl->dev_stripes;
> >   }
> >
> > @@ -5143,10 +5143,9 @@ static void init_alloc_chunk_ctl_policy_zoned(
> >   		BUG();
> >   	}
> >
> > -	/* We don't want a chunk larger than 10% of writable space */
> > -	limit = max(round_down(div_factor(fs_devices->total_rw_bytes, 1),
> > -			       zone_size),
> > -		    min_chunk_size);
> > +	/* We don't want a chunk larger than 1/8 of writable space */
> > +	limit = max_t(u64, min_chunk_size,
> > +		rounddown_pow_of_two(fs_devices->total_rw_bytes >> 3));
> >   	ctl->max_chunk_size = min(limit, ctl->max_chunk_size);
> >   	ctl->dev_extent_min = zone_size * ctl->dev_stripes;
> >   }
> > @@ -5284,13 +5283,12 @@ static int decide_stripe_size_regular(struct alloc_chunk_ctl *ctl,
> >   	 */
> >   	if (ctl->stripe_size * data_stripes > ctl->max_chunk_size) {
> >   		/*
> > -		 * Reduce stripe_size, round it up to a 16MB boundary again and
> > +		 * Reduce stripe_size, round down to pow of 2 boundary again and
> >   		 * then use it, unless it ends up being even bigger than the
> >   		 * previous value we had already.
> >   		 */
> > -		ctl->stripe_size = min(round_up(div_u64(ctl->max_chunk_size,
> > -							data_stripes), SZ_16M),
> > -				       ctl->stripe_size);
> > +		ctl->stripe_size = min_t(u64, ctl->stripe_size,
> > +			rounddown_pow_of_two(div_u64(ctl->max_chunk_size, data_stripes)));
> 
> I think this can even be problematic since now stripe_size can be much
> smaller than what we want.
> 
> Thanks,
> Qu

rounddown_pow_of_two() is at least > round_up(,SZ_16M) * 0.5,
so it is not too much smaller.

But  roundup_pow_of_two() match with orig round_up(,SZ_16M)
better than rounddown_pow_of_two().

It will be changed to roundup_pow_of_two() in v2.

Best Regards
Wang Yugui (wangyugui@e16-tech.com)
2022/08/16