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Bug#584881: Lockups under heavy disk IO; md (RAID) resync/check implicated

On Thu, 2010-06-24 at 11:17 +0100, Ian Jackson wrote:
> Ben Hutchings writes ("Re: Bug#584881: Lockups under heavy disk IO; md (RAID) resync/check implicated"):
> > > Search the web suggests that symptoms very similar to mine are not
> > > uncommon, including instances without soft lockup messages, and none
> > > of the other users seem to have a similar disk layout.
> > > 
> > > I can't easily test this theory but I think the unusual disk layout is
> > > probably simply making a race easier to trigger.
> > 
> > Thinking of some kind of lock-dependency bug?  Blocking on a mutex for a
> > long period should still trigger a soft-lockup message.  Since there are
> > no messages from the kernel it's something of a mystery what's going on.
> The RAID system (md driver) has a separate mechanism for blocking
> writes, which it calls a "barrier".  I think it is quite capable of
> indefinitely blocking all writes to a device without necessarily
> triggering the soft lockup detector.

I/O barriers are block I/O operations (not specific to md) that inhibit
reordering of read and write operations.  They certainly should not be
blocking operations.  Also, device-mapper did not support barriers until
after 2.6.26 so md will not be using them in the configuration you are


Ben Hutchings
Once a job is fouled up, anything done to improve it makes it worse.

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