[CentOS-virt] I/O load distribution

Mon Jul 27 13:17:46 UTC 2009
Manuel Wolfshant <wolfy at nobugconsulting.ro>

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.
In this particular case, I would edit /etc/crontab on each VM and move 
the update at another time instead of the traditional 4:02

> 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?
> Regards,
>    Dennis