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Re: [Nbd] deadlock in nbd?

On 10/22/07, Eric Gerlach <egerlach@...135...> wrote:
> Peter Daum wrote:
> > Hi,
> >
> > Mike Snitzer <snitzer@...17...>  wrote:
> >  > Which IO scheduler are you using?  Please have a look at this redhat
> >> bug; turns out that there is definitely an issue with cfq (not
> >> exclussive to redhat):
> >> https://bugzilla.redhat.com/show_bug.cgi?id=241540
> >  >
> >> But if you're seeing this on stock kernel.org I'd imagine your just
> >> using anticipatory.
> >
> > ... I'll do some more thorough testing to be sure that this was indeed all of it,
> > but at first sight it looks like this is also what is striking me - On all machines
> > but one (accidentally also the only one without interface bonding) I had "cfq"
> > as the default IO scheduler. After settting the scheduler for /deb/nbd0 to "noop",
> > so far it is running without a problem ...
> Ha ha!  It works!  Thanks a lot for the pointer.  For future reference
> to other people who may not want to go on a search, the secret password is:
> echo noop > /sys/block/nbdX/queue/scheduler (replace X as necessary).
> I have only one further question:  Is it safe to change the scheduler
> while the device is mounted?  I'd like to put that command in my
> rc.local, but it will only run after the device is mounted.

Actually it was pointed out (later in the redhat bugziila, by David
Miller) that using the noop scheduler for nbd really isn't too smart.
That is because the IO requests aren't coalesced before they're sent
out over the wire via nbd.  As a result nbd becomes more inefficient.
Using cfq is fine provided you patch the kernel (like redhat has
done).  I've been using deadline with good success.

And its perfectly safe to change the IO scheduler while the device is
mounted.  But as I said, you really do want to use one of the
schedulers.  So just changing the system default via elevator=<blah>
on the Linux kernel command line may be sufficient.


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