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Re: [Linux-cluster] GFS2 processes getting stuck in WCHAN=dlm_posix_lock



Hi Again,

On 11/02/2009 06:42 AM, Steven Whitehouse wrote:
Hi,

On Fri, 2009-10-30 at 19:27 -0400, Allen Belletti wrote:
Hi All,

As I've mentioned before, I'm running a two-node clustered mail server
on GFS2 (with RHEL 5.4)  Nearly all of the time, everything works
great.  However, going all the way back to GFS1 on RHEL 5.1 (I think it
was), I've had occasional locking problems that force a reboot of one or
both cluster nodes.  Lately I've paid closer attention since it's been
happening more often.

I'll notice the problem when the load average starts rising.  It's
always tied to "stuck" processes, and I believe always tied to IMAP
clients (I'm running Dovecot.)  It seems like a file belonging to user
"x" (in this case, "jforrest" will become locked in some way, such that
every IMAP process tied that user will get stuck on the same thing.
Over time, as the user keeps trying to read that file, more&  more
processes accumulate.  They're always in state "D" (uninterruptible
sleep), and always on "dlm_posix_lock" according to WCHAN.  The only way
I'm able to get out of this state is to reboot.  If I let it persist for
too long, I/O generally stops entirely.

This certainly seems like it ought to have a definite solution, but I've
no idea what it is.  I've tried a variety of things using "find" to
pinpoint a particular file, but everything belonging to the affected
user seems just fine.  At least, I can read and copy all of the files,
and do a stat via ls -l.

Is it possible that this is a bug, not within GFS at all, but within
Dovecot IMAP?

Any thoughts would be appreciated.  It's been getting worse lately and
thus no fun at all.

Cheers,
Allen

Do you know if dovecot IMAP uses signals at all? That would be the first
thing that I'd look at. The other thing to check is whether it makes use
of F_GETLK and in particular the l_pid field? strace should be able to
answer both of those questions (except the l_pid field of course, but
the chances are it it calls F_GETLK and then sends a signal, its also
using the l_pid field),

Steve.
I've been looking into how Dovecot IMAP works and I see now that no "locking" in the OS sense of the word is involved for Maildir access. Instead, one particular index per mail folder is "locked" by creating a <filename>.lock entry, performing the necessary operations, and then deleting the file. In the case of certain users with hundreds of folders and mail clients which scan all of them, this potentially results in hundreds of rapid create/delete operations. The relevant text from the Dovecot documentation is as follows:

Although maildir was designed to be lockless, Dovecot locks the maildir while
doing modifications to it or while looking for new messages in it. This is
required because otherwise Dovecot might temporarily see mails incorrectly
deleted, which would cause trouble. Basically the problem is that if one
process modifies the maildir (eg. a rename() to change a message's flag),
another process in the middle of listing files at the same time could skip a file. The skipping happens because readdir() system call doesn't guarantee that all the files are returned if the directory is modified between the calls to
it. This problem exists with all the commonly used filesystems.

Because Dovecot uses its own non-standard locking ('dovecot-uidlist.lock'
dotlock file), other MUAs accessing the maildir don't support it. This means that if another MUA is updating messages' flags or expunging messages, Dovecot
might temporarily lose some message. After the next sync when it finds it
again, an error message may be written to log and the message will receive a
new UID.
Does GFS2 have the limitation that's being described for readdir()? I would expect so, but perhaps the work necessary to ensure a consistent view between cluster node has the side effect of correcting this issue as well. In any case, the number of times that my users would actually encounter the issue being protected against might be so rare that I can safely disable the locking mechanism regardless.

Any thoughts on this would be appreciated. Would this sequence of operations cause the WCHAN=dlm_posix_lock condition for brief periods of time in normal operation? Wish I could dig through the kernel & gfs2 code to figure this out for myself but it would crush my productivity at work :-)

Cheers,
Allen

--
Allen Belletti
allen isye gatech edu                             404-894-6221 Phone
Industrial and Systems Engineering                404-385-2988 Fax
Georgia Institute of Technology


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