Re: [PATCH 2/2] Btrfs: make deduplication with range including the last block work

[Josef Bacik <josef@toxicpanda.com>]

On 12/16/19 1:26 PM, fdmanana@kernel.org wrote:
> From: Filipe Manana <fdmanana@suse.com>
> 
> Since btrfs was migrated to use the generic VFS helpers for clone and
> deduplication, it stopped allowing for the last block of a file to be
> deduplicated when the source file size is not sector size aligned (when
> eof is somewhere in the middle of the last block). There are two reasons
> for that:
> 
> 1) The generic code always rounds down, to a multiple of the block size,
>     the range's length for deduplications. This means we end up never
>     deduplicating the last block when the eof is not block size aligned,
>     even for the safe case where the destination range's end offset matches
>     the destination file's size. That rounding down operation is done at
>     generic_remap_check_len();
> 
> 2) Because of that, the btrfs specific code does not expect anymore any
>     non-aligned range length's for deduplication and therefore does not
>     work if such nona-aligned length is given.
> 

Does anybody else rely on this behavior that needs a change like this for their fs?

> This patch addresses that second part, and it depends on a patch that
> fixes generic_remap_check_len(), in the VFS, which was submitted ealier
> and has the following subject:
> 
>    "fs: allow deduplication of eof block into the end of the destination file"
> 
> These two patches address reports from users that started seeing lower
> deduplication rates due to the last block never being deduplicated when
> the file size is not aligned to the filesystem's block size.
> 
> Link: https://lore.kernel.org/linux-btrfs/2019-1576167349.500456@svIo.N5dq.dFFD/
> Signed-off-by: Filipe Manana <fdmanana@suse.com>

Reviewed-by: Josef Bacik <josef@toxicpanda.com>

Thanks,

Josef