Re: xfs log error: head behind tail

[Dave Chinner <david@fromorbit.com>]

On Tue, Mar 13, 2012 at 06:07:56PM +0000, Pedro Ribeiro wrote:
> Hi,
> 
> I'm running a custom kernel in one of my production systems. It is
> basically a 2.6.39.4 patched with TuxOnIce and backporting of a number
> of xfs and vfs patches from the 3.0 series:
> xfs-avoid-direct-i-o-write-vs-buffered-i-o-race.patch
> xfs-avoid-synchronous-transactions-when-deleting-attr-blocks.patch
> xfs-do-not-update-xa_last_pushed_lsn-for-locked-items.patch
> xfs-dont-serialise-direct-io-reads-on-page-cache.patch
> xfs-fix-attr2-vs-large-data-fork-assert.patch
> xfs-fix-buffer-flushing-during-unmount.patch
> xfs-fix-error-handling-for-synchronous-writes.patch
> xfs-fix-nfs-export-of-64-bit-inodes-numbers-on-32-bit-kernels.patch
> xfs-fix-possible-memory-corruption-in-xfs_readlink.patch
> xfs-fix-write_inode-return-values.patch
> xfs-fix-xfs_mark_inode_dirty-during-umount.patch
> xfs-force-buffer-writeback-before-blocking-on-the-ilock-in-inode-reclaim.patch
> xfs-force-the-log-if-we-encounter-pinned-buffers-in-.iop_pushbuf.patch
> xfs-return-eio-when-xfs_vn_getattr-failed.patch
> xfs-revert-to-using-a-kthread-for-ail-pushing-botto.patch
> xfs-start-periodic-workers-later.patch
> xfs-use-a-cursor-for-bulk-ail-insertion.patch
> xfs-use-doalloc-flag-in-xfs_qm_dqattach_one.patch
> xfs-validate-acl-count.patch
> vfs-add-device-tag-to-proc-self-mountstats.patch
> vfs-automount-should-ignore-lookup_follow.patch
> vfs-fix-automount-for-negative-autofs-dentries.patch
> vfs-fix-statfs-automounter-semantics-regression.patch
> vfs-fix-the-remaining-automounter-semantics-regressions.patch
> vfs-pathname-lookup-add-lookup_automount-flag.patch
> vfs-show-o_cloexe-bit-properly-in-proc-pid-fdinfo-fd-files.patch
> (among other non-fs backported patches)
> 
> 
> My syslog is showing a lot of these messages:
> 
> Mar 13 18:01:49 Biramilho kernel: [509425.318618] XFS (dm-1):
> xlog_space_left: head behind tail
> Mar 13 18:01:49 Biramilho kernel: [509425.318620]   tail_cycle = 345,
> tail_bytes = 103334400
> Mar 13 18:01:49 Biramilho kernel: [509425.318623]   GH   cycle = 345,
> GH   bytes = 103334192
> Mar 13 18:05:56 Biramilho kernel: [509672.560893] XFS (dm-1):
> xlog_space_left: head behind tail
> Mar 13 18:05:56 Biramilho kernel: [509672.560897]   tail_cycle = 345,
> tail_bytes = 103366144
> Mar 13 18:05:56 Biramilho kernel: [509672.560900]   GH   cycle = 345,
> GH   bytes = 103365936
> Mar 13 18:05:56 Biramilho kernel: [509672.560911] XFS (dm-1):
> xlog_space_left: head behind tail
> Mar 13 18:05:56 Biramilho kernel: [509672.560914]   tail_cycle = 345,
> tail_bytes = 103366144
> Mar 13 18:05:56 Biramilho kernel: [509672.560917]   GH   cycle = 345,
> GH   bytes = 103365936
> 
> dm-1 is a partition in a LVM device inside a LUKS encrypted container.
> My uptime is about 67 days with 70 TuxOnIce hibernaitons in between.
> This started to happen at least 10 days ago.

TuxOnIce freezes filesystems during hibernation, doesn't it? We've
recently had a report of this problem to do with a test that
repeatedly froze a filesystem, and it looked like the freeze was
leaking 8 bytes of log space on every freeze/thaw cycle. This is
probably the same issue.

> Is this dangerous? Should I be worried?

Not particularly dangerous, though I'd suggest a unmount/mount on
the filesystem to get the head and tail back in sync.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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