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[PATCH 3/4] xfs: limit speculative prealloc near ENOSPC thresholds

To: xfs@xxxxxxxxxxx
Subject: [PATCH 3/4] xfs: limit speculative prealloc near ENOSPC thresholds
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Mon, 21 Jan 2013 23:53:54 +1100
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <1358772835-21436-1-git-send-email-david@xxxxxxxxxxxxx>
References: <1358772835-21436-1-git-send-email-david@xxxxxxxxxxxxx>
From: Dave Chinner <dchinner@xxxxxxxxxx>

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@xxxxxxxxxx>
---
 fs/xfs/xfs_iomap.c |    9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
index 3587772..de3bad5 100644
--- a/fs/xfs/xfs_iomap.c
+++ b/fs/xfs/xfs_iomap.c
@@ -395,6 +395,15 @@ 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)
-- 
1.7.10

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