Re: [PATCH v3]btrfs: round down stripe size and chunk size to pow of 2

[Wang Yugui <wangyugui@e16-tech.com>]

Hi,

> On 2022/8/17 22:58, Wang Yugui wrote:
> > In decide_stripe_size_regular(), when new disk is added to RAID0/RAID10/RAID56,
> > it is better to free-then-reuse the free space if stripe size is kept or
> > changed to 1/2. so stripe size of pow of 2 will be more friendly. Although
> > roundup_pow_of_two() match better with orig round_up(), but
> > rounddown_pow_of_two() is better to make sure <=ctl->max_chunk_size here.
> 
> Why insist on round*_pow_of_two()?
> 
> I see no reason that a power of 2 sized chunk has any benefit to btrfs.

For stripe size, in some case, 
decide_stripe_size_regular()
    if (ctl->stripe_size * data_stripes > ctl->max_chunk_size) {
        /*
         * Reduce stripe_size, round it up to a 16MB 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);
    }

For example, RAID0/3 disk/max_chunk_size=1G,
then stripe_size = about 1/3G

If another disk is added, RAID0/4 disk/max_chunk_size=1G,
then stripe_size = 1/4G

the mix of 1/3G and 1/4G stripe_size is difficult to manage alloc/free
the space than the mix of 1/2G and 1/4G .

For chunk size, it is more complex because of raid profile.
decide_stripe_size_regular()
    ctl->chunk_size = ctl->stripe_size * data_stripes;
'
for some raid proflie  such as single/RAID1,
because stripe size is already set to a power of 2,
if we set max_chunk_size to  a power of 2, then max_chunk_size will have
a value same to chunk size/ stripe size in some case. it will be more
easy to understand.

Best Regards
Wang Yugui (wangyugui@e16-tech.com)
2022/08/18


> >
> > In another rare case that file system is quite small, we calc max chunk size
> > in pow of 2 too, so that max chunk size / chunk size /stripe size are same or
> > match easy in some case.
> >
> > Signed-off-by: Wang Yugui <wangyugui@e16-tech.com>
> > ---
> > changes since v2:
> > 	restore to rounddown_pow_of_two() from roundup_pow_of_two()
> > changes since v1:
> >   - change rounddown_pow_of_two() to roundup_pow_of_two() to match better with
> >     orig roundup().
> >
> >   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 it 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 */