Re: kvm, drbd, elevator, rotational - quite an interesting co-operation

Javier Guerra
2009-07-02T20:14:34Z

On Thu, Jul 2, 2009 at 2:55 PM, Michael Tokarev<mjt@tls.msk.ru> wrote:
> kvm: i/o threads - should there be a way to control the amount of
>  threads?  With default workload generated by drbd on secondary
>  node having less thread makes more sense.

+1 on this. it seems reasonable to have one thread per device, or am
i wrong?

it also bothers me because when i have a couple of moderately
disk-heavy VMs, the load average numbers skyrockets. that's because
each blocked thread counts as 1 on this figure, even if they're all
waiting on the same device.

> kvm: it has been said that using noop elevator on guest makes sense
>  since host does its own elevator/reordering.  But this example
>  shows "nicely" that this isn't always the case.  I wonder how
>  "general" this example is.  Will try to measure further.

on my own (quick) tests, changing the elevator on the guest has very
little effect on performance; but does affect the host CPU
utilization. using drbd on the guest while testing with bonnie++
increased host CPU by around 20% for each VM

--
Javier