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Re: kernel: XFS (dm-7): xlog_space_left: head behind tail

To: Gregory Machin <gdm@xxxxxxxxxxxxxx>
Subject: Re: kernel: XFS (dm-7): xlog_space_left: head behind tail
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Wed, 21 Mar 2012 16:20:27 +1100
Cc: xfs@xxxxxxxxxxx
In-reply-to: <20120321043031.GW5091@dastard>
References: <CAJzjPKkUm-uWuGs2xPLF9yZ71uUV8QwPApjGAf6EKDoGqQYqBA@xxxxxxxxxxxxxx> <20120321043031.GW5091@dastard>
User-agent: Mutt/1.5.21 (2010-09-15)
On Wed, Mar 21, 2012 at 03:30:31PM +1100, Dave Chinner wrote:
> On Wed, Mar 21, 2012 at 05:00:02PM +1300, Gregory Machin wrote:
> > I have a CentOS 6.2 virtual machine in a vmware ESXi 4.0 host 4G Ram 4
> > virtaul cpus and with about 4 TB of disk space formatted with XFS .
> > I'm seeing a lot of :
> > 
> > Mar 21 10:49:42 nzhmlfpr05 kernel: XFS (dm-4): xlog_space_left: head behind 
> > tail
> > Mar 21 10:49:42 nzhmlfpr05 kernel:  tail_cycle = 129, tail_bytes = 20163072
> > Mar 21 10:49:42 nzhmlfpr05 kernel:  GH   cycle = 129, GH   bytes = 20162880
> > Mar 21 10:49:42 nzhmlfpr05 kernel: XFS (dm-7): xlog_space_left: head behind 
> > tail
> > Mar 21 10:49:42 nzhmlfpr05 kernel:  tail_cycle = 5, tail_bytes = 333417984
> > Mar 21 10:49:42 nzhmlfpr05 kernel:  GH   cycle = 5, GH   bytes = 333417792
> > Mar 21 10:49:42 nzhmlfpr05 kernel: XFS (dm-7): xlog_space_left: head behind 
> > tail
> > Mar 21 10:49:42 nzhmlfpr05 kernel:  tail_cycle = 5, tail_bytes = 333417984
> > Mar 21 10:49:42 nzhmlfpr05 kernel:  GH   cycle = 5, GH   bytes = 333417792
> 
> can you send the complete set of output, including the first
> occurrence of it?
> 
> What workload are you running? Are you freezing you filesystem
> frequently?

If you are freezing regularly, then the patch below should fix the
problem.

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx

xfs: Account log unmount transaction correctly

From: Dave Chinner <dchinner@xxxxxxxxxx>

There have been a few reports of this warning appearing recently:

XFS (dm-4): xlog_space_left: head behind tail
 tail_cycle = 129, tail_bytes = 20163072
 GH   cycle = 129, GH   bytes = 20162880

The common cause appears to be lots of freeze and unfreeze cycles,
and the output from the warnings indicates that we are leaking
around 8 bytes of log space per freeze/unfreeze cycle.

When we freeze the filesystem, we write an unmount record and that
uses xlog_write directly - a special type of transaction,
effectively. What it doesn't do, however, is correctly acocunt for
the log space it uses. The unmount record writes an 8 byte structure
with a special magic number into the log, and the space this
consumes is not accounted for in the log ticket tracking the
operation. Hence we leak 8 bytes every unmount record that is
written.

Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
---
 fs/xfs/xfs_log.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)

diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
index 98a9cb5..6db1fef 100644
--- a/fs/xfs/xfs_log.c
+++ b/fs/xfs/xfs_log.c
@@ -726,8 +726,9 @@ xfs_log_unmount_write(xfs_mount_t *mp)
                                .lv_iovecp = &reg,
                        };
 
-                       /* remove inited flag */
+                       /* remove inited flag, and account for space used */
                        tic->t_flags = 0;
+                       tic->t_curr_res -= sizeof(magic);
                        error = xlog_write(log, &vec, tic, &lsn,
                                           NULL, XLOG_UNMOUNT_TRANS);
                        /*

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