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Re: [Xen-users] Disk starvation between DomU's

  • To: Wiebe Cazemier <wiebe@xxxxxxxxxxxx>
  • From: Thanos Makatos <thanos.makatos@xxxxxxxxxx>
  • Date: Wed, 19 Jun 2013 16:09:21 +0000
  • Accept-language: en-GB, en-US
  • Cc: "xen-users@xxxxxxxxxxxxx" <xen-users@xxxxxxxxxxxxx>
  • Delivery-date: Wed, 19 Jun 2013 16:10:09 +0000
  • List-id: Xen user discussion <xen-users.lists.xen.org>
  • Thread-index: sntHw+yzxcSzFuitBwG/fIiMFfnXdaY7z0TggAG4oICACbz9AIACt41QR8OB4sT9wkGE8MOzfjtPvEyd3hA=
  • Thread-topic: [Xen-users] Disk starvation between DomU's

> Hmm. That didn't make a difference, but something else did. I was
> looking at the read column... Stupid mistake, but not so stupid as you
> might think. My eye was drawn to the changing figures, and now I see
> that write IO is always 0, according to iotop. When I do "iotop -oa"
> (accumulated, leave out non-active processes), all blkback processes
> that are appearing all accumulate 0 bytes written. I don't understand
> that...

Can you try "iostat -x 1" instead of iotop?

> That's correct. Currently, the RAID array is CFQ. I would seem weird to
> me to change that into noop. The RAID controller might schedule, but it
> can't receive instructions from the OS what should have priority. I'll
> look into it, though.

The rule of thumb is to let the RAID controller do the scheduling; otherwise 
the two schedulers may end "competing" with each other. Of course this depends 
on the RAID controller, the I/O workload etc. so it may make no difference in 
your particular case.

> I do know that the recommended DomU scheduler is noop. It's also the
> default for all my machines without configuring it. I guess they know
> they're virtual.

Not necessarily: using CFQ inside a VM would still make sense if you want to 
enforce I/O fairness among the applications running inside it, although this 
could potentially lead to weird interactions with the OS's/controller's I/O 

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