btrfs: skip hole detection during full fsync for files without holes

Browse source

If we the no-holes feature is enabled (a default since btrfs-progs 5.15),
when doing a full fsync we always iterate of all leaves in the subvolume
root that contain file extent items in order to detect holes between them.

This can take a lot of time for files with a large number of extents.
But if we know there are no prealloc extents and the amount of space
(uncompressed space) is greater than or equals to the i_size of the
inode, then we cannot have holes and therefore avoid searching for
them. So skip the search if those conditions are met.

The following test script was used:

  $ cat test.sh
  #!/bin/bash

  MNT=/mnt/nullb0
  DEV=/dev/nullb0

  umount $MNT &> /dev/null
  mkfs.btrfs -f $DEV
  mount $DEV $MNT

  # 256M gives 64K extents of 4K each.
  FILE_SIZE=$((256 * 1024 * 1024))
  touch $MNT/foobar

  for ((i = 0; i < $FILE_SIZE; i += 8192)); do
      xfs_io -c "pwrite -S 0xab $i 4K" $MNT/foobar > /dev/null
  done

  xfs_io -c "fsync" $MNT/foobar

  for ((i = 4096; i < $FILE_SIZE; i += 8192)); do
     xfs_io -c "pwrite -S 0xab $i 4K" $MNT/foobar > /dev/null
  done

  # unmount and mount, clear caches and ensure the next fsync is a
  # full sync.
  umount $MNT
  mount $DEV $MNT

  # Do some change to the file in order to fsync.
  xfs_io -c "pwrite -S 0xcd 0 4K" $MNT/foobar > /dev/null

  T0=$(date +%s%N)
  xfs_io -c "fsync" $MNT/foobar
  T1=$(date +%s%N)

  echo
  echo "Took $(( (T1 - T0) / 1000 ))us"

  umount $MNT

Before this change:

  Took 28721us

After this change:

  Took 5453us

That's about 5.3x times faster.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

...

This commit is contained in:

Filipe Manana 2026-07-30 16:31:35 +01:00 • committed by David Sterba

parent 6003c20dbd

commit 4096e2a06f

1 changed files with 9 additions and 0 deletions

Show all changes Ignore whitespace when comparing lines Ignore changes in amount of whitespace Ignore changes in whitespace at EOL

Download patch file Download diff file Expand all files Collapse all files

9

fs/btrfs/tree-log.c

View file