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ext4: flush delalloc blocks when space is low

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Creating many small files in rapid succession on a small
filesystem can lead to spurious ENOSPC; on a 104MB filesystem:

for i in `seq 1 22500`; do
    echo -n > $SCRATCH_MNT/$i
    echo XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX > $SCRATCH_MNT/$i
done

leads to ENOSPC even though after a sync, 40% of the fs is free
again.

This is because we reserve worst-case metadata for delalloc writes,
and when data is allocated that worst-case reservation is not
usually needed.

When freespace is low, kicking off an async writeback will start
converting that worst-case space usage into something more realistic,
almost always freeing up space to continue.

This resolves the testcase for me, and survives all 4 generic
ENOSPC tests in xfstests.

We'll still need a hard synchronous sync to squeeze out the last bit,
but this fixes things up to a large degree.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
This commit is contained in:
Eric Sandeen 2009-12-23 07:58:12 -05:00 committed by Theodore Ts'o
parent 17bd55d037
commit c8afb44682
1 changed files with 9 additions and 2 deletions
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11
fs/ext4/inode.c
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@ -2993,11 +2993,18 @@ static int ext4_nonda_switch(struct super_block *sb)
if (2 * free_blocks < 3 * dirty_blocks ||
free_blocks < (dirty_blocks + EXT4_FREEBLOCKS_WATERMARK)) {
/*
* free block count is less that 150% of dirty blocks
* or free blocks is less that watermark
* free block count is less than 150% of dirty blocks
* or free blocks is less than watermark
*/
return 1;
}
/*
* Even if we don't switch but are nearing capacity,
* start pushing delalloc when 1/2 of free blocks are dirty.
*/
if (free_blocks < 2 * dirty_blocks)
writeback_inodes_sb_if_idle(sb);
return 0;
}