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xfs: limit speculative prealloc near ENOSPC thresholds

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There is a window on small filesytsems where specualtive
preallocation can be larger than that ENOSPC throttling thresholds,
resulting in specualtive preallocation trying to reserve more space
than there is space available. This causes immediate ENOSPC to be
triggered, prealloc to be turned off and flushing to occur. One the
next write (i.e. next 4k page), we do exactly the same thing, and so
effective drive into synchronous 4k writes by triggering ENOSPC
flushing on every page while in the window between the prealloc size
and the ENOSPC prealloc throttle threshold.

Fix this by checking to see if the prealloc size would consume all
free space, and throttle it appropriately to avoid premature
ENOSPC...

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Ben Myers <bpm@sgi.com>
This commit is contained in:
Dave Chinner 2013-01-21 23:53:54 +11:00 committed by Ben Myers
parent 10616b806d
commit 4d559a3bcb
1 changed files with 9 additions and 0 deletions
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9
fs/xfs/xfs_iomap.c
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@ -351,6 +351,15 @@ xfs_iomap_prealloc_size(
}
if (shift)
alloc_blocks >>= shift;
/*
* If we are still trying to allocate more space than is
* available, squash the prealloc hard. This can happen if we
* have a large file on a small filesystem and the above
* lowspace thresholds are smaller than MAXEXTLEN.
*/
while (alloc_blocks >= freesp)
alloc_blocks >>= 4;
}
if (alloc_blocks < mp->m_writeio_blocks)