Re: [fedora-virt] I/O load distribution
On Mon, Jul 27, 2009 at 07:22:27PM +0300, Pasi Kärkkäinen wrote:
On Mon, Jul 27, 2009 at 03:15:05PM +0200, Dennis J. wrote:
> Hi,
> What is the best way to deal with I/O load when running several VMs on a
> physical machine with local or remote storage?
>
> What I'm primarily worried about is the case when several VMs cause disk
> I/O at the same time. One example would be the "updatedb" cronjob of the
> mlocate package. If you have say 5 VMs running on a physical System with a
> local software raid-1 as storage and the all run updatedb at the same time
> that causes all of them to run really slowly because the starve each other
> fighting over the disk.
>
> What is the best way to soften the impact of such a situation? Does it make
> sense to use a hardware raid instead? How would the raid type affect the
> performance in this case? Would the fact that the I/O load gets distributed
> across multiple spindles in, say, a 4 disk hardware raid-5 have a big
> impact on this?
>
> I'm currently facing the problem where I fear that random disk I/O by too
> many VMs on a physical system could cripple their performance even though I
> have plenty of CPU cores/RAM left to run them.
>
> Has anyone experience with this problem and maybe some data to shed some
> light on this potential bottleneck for virtualization?
>
Well, there's only so much IOPS you can get from a single disk.. (or raid1).
IO generated from multiple virtual machines is pretty random, so you just
need a lot of disks to make it faster.
Single 7200 rpm SATA disk will give you only 100-150 IOPS (IOs per second)
max.. 10 disks will give you 1000-1500 IOPS max.. it's pretty easy maths.
.. Or then get faster disks. Single 15k rpm SAS disk will give you up to 350
IOPS, so it's 2-3x faster than 7200 rpm SATA.
-- Pasi