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

Javier Guerra
2009-07-06T14:07:24Z

On Mon, Jul 6, 2009 at 8:28 AM, Michael Tokarev<mjt@tls.msk.ru> wrote:
> Javier Guerra wrote:
>> 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.
>
> And how having large LA is bad?  I mean, LA by itself is not an
> indicator of bad or good performance, don't you think?

it's not a real problem, of course; but it's a nuisance because some
reporting tools (zabbix/nessus) use this figure to raise alarms,
meaning i have to adjust it.

also, even a single-threaded high-IO process on a guest fires a lot of
IO threads on the host, and other not-so-aggressive VMs suffer.

definitely using deadline scheduler on the host reduces the impact.
(down to zero? i don't think so, but it's certainly manageable)

>> 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
>
> Increased compared what with what?  Also, which virtual disk format
> did you use?

sorry, i had a typo there, i meant: using cfq vs. noop on the guest
(running bonnie++, no drbd anywhere) produced around 20% more CPU load
on the host, with no measurable performance advantage.

--
Javier