Commit c8afb446 authored Dec 23, 2009 by Eric Sandeen Committed by Theodore Ts'o Dec 23, 2009

ext4: flush delalloc blocks when space is low



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>

parent 17bd55d0

fs/ext4/inode.c

+9 −2

Original line number	Diff line number	Diff line
		@@ -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;
		}