Re: [PATCH v2] btrfs: scrub: avoid unnecessary extent tree search for striped profiles

[David Sterba <dsterba@suse.cz>]

On Tue, Aug 15, 2023 at 07:07:19PM +0800, Qu Wenruo wrote:
> [PROBLEM]
> Since commit 8557635ed2b0 ("btrfs: scrub: introduce dedicated helper to
> scrub simple-stripe based range"), the scrub speed of striped profiles
> (RAID0/RAID10/RAID5/RAID6) are degraded, if the block group is mostly
> empty or fragmented.
> 
> [CAUSE]
> In scrub_simple_stripe(), which is the responsible for RAID0/RAID10
> profiles, we just call scrub_simple_mirror() and increase our
> @cur_logical and @cur_physical.
> 
> The problem is, if there are no more extents inside the block group, or
> the next extent is far away from our current logical, we would call
> scrub_simple_mirror() for the empty ranges again and again, until we
> reach the next next.
> 
> This is completely a waste of CPU time, thus it greatly degrade the
> scrub performance for stripped profiles.
> 
> This is also affecting RAID56, as we rely on scrub_simple_mirror() for
> data stripes of RAID56.
> 
> [FIX]
> - Introduce scrub_ctx::found_next to record the next extent we found
>   This member would be updated by find_first_extent_item() calls inside
>   scrub_find_fill_first_stripe().
> 
> - Skip to the next stripe directly in scrub_simple_stripe()
>   If we detect sctx->found_next is beyond our current stripe, we just
>   skip to the full stripe which covers the target bytenr.
> 
> - Skip to the next full stripe covering sctx->found_next
>   Unlike RAID0/RAID10, we can not easily skip to the next stripe due to
>   rotation.
>   But we can still skip to the next full stripe, which can still save us
>   a lot of time.
> 
> Fixes: 8557635ed2b0 ("btrfs: scrub: introduce dedicated helper to scrub simple-stripe based range")
> Signed-off-by: Qu Wenruo <wqu@suse.com>
> ---
> Changelog:
> v2:
> - Fix a u64/u32 division not using the div_u64() helper
> 
> - Slightly change the advancement of logical/physical for RAID0 and
>   RAID56
>   Now logical/physical is always increased first, this removes one
>   if () branch.
> 
> This patch is based on the scrub_testing branch (which is misc-next +
> scrub performance fixes).
> 
> Thus there would be quite some conflicts for stable branches and would
> need manual backport.

Added to misc-next, thanks.