Re: [PATCH] btrfs/280: run defrag after creating file to get expected extent layout

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

在 2024/6/5 20:56, fdmanana@kernel.org 写道:
> From: Filipe Manana <fdmanana@suse.com>
>
> The test writes a 128M file and expects to end up with 1024 extents, each
> with a size of 128K, which is the maximum size for compressed extents.
> Generally this is what happens, but often it's possibly for writeback to
> kick in while creating the file (due to memory pressure, or something
> calling sync in parallel, etc) which may result in creating more and
> smaller extents, which makes the test fail since its golden output
> expects exactly 1024 extents with a size of 128K each.
>
> So to work around run defrag after creating the file, which will ensure
> we get only 128K extents in the file.

But defrag is not much different than reading the page and set it dirty
for writeback again.

It can be affected by the same memory pressure things to get split.

I guess you choose compressed file extents is to bump up the subvolume
tree meanwhile also compatible for all sector sizes.

In that case, what about doing DIO using sectorsize of the fs?
So that each dio write would result one file extent item, meanwhile
since it's a single sector/page, memory pressure will never be able to
writeback that sector halfway.

Thanks,
Qu
>
> Signed-off-by: Filipe Manana <fdmanana@suse.com>
> ---
>   tests/btrfs/280 | 10 +++++++++-
>   1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/tests/btrfs/280 b/tests/btrfs/280
> index d4f613ce..0f7f8a37 100755
> --- a/tests/btrfs/280
> +++ b/tests/btrfs/280
> @@ -13,7 +13,7 @@
>   # the backref walking code, used by fiemap to determine if an extent is shared.
>   #
>   . ./common/preamble
> -_begin_fstest auto quick compress snapshot fiemap
> +_begin_fstest auto quick compress snapshot fiemap defrag
>
>   . ./common/filter
>   . ./common/punch # for _filter_fiemap_flags
> @@ -36,6 +36,14 @@ _scratch_mount -o compress
>   # extent tree (if the root was a leaf, we would have only data references).
>   $XFS_IO_PROG -f -c "pwrite -b 1M 0 128M" $SCRATCH_MNT/foo | _filter_xfs_io
>
> +# While writing the file it's possible, but rare, that writeback kicked in due
> +# to memory pressure or a concurrent sync call for example, so we may end up
> +# with extents smaller than 128K (the maximum size for compressed extents) and
> +# therefore make the test expectations fail because we get more extents than
> +# what the golden output expects. So run defrag to make sure we get exactly
> +# the expected number of 128K extents (1024 extents).
> +$BTRFS_UTIL_PROG filesystem defrag "$SCRATCH_MNT/foo" >> $seqres.full
> +
>   # Create a RW snapshot of the default subvolume.
>   _btrfs subvolume snapshot $SCRATCH_MNT $SCRATCH_MNT/snap
>