[fedora-virt] I/O load distribution

Dennis J. dennisml at conversis.de
Mon Jul 27 13:15:05 UTC 2009

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?


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