[CentOS] Swap space for kvm virtual host
brian.mathis at gmail.com
Tue Mar 15 13:48:25 UTC 2011
On Mon, Mar 14, 2011 at 5:24 PM, Nataraj <incoming-centos at rjl.com> wrote:
> I have a kvm virtual host running on what will become CentOS 6 with 12GB of
> memory and a Quad Xeon X5560 2.8Ghz . The store for virtual machines will
> be a software raid 6 array of 6 disks with an LVM layered on top. I'm not
> initially planning any major overcommitment of resources, though there could
> be a need for some overcommitment with a light workload on the guests.
> In recent years people seem to configure a wide range of different swap
> allocations. I was thinking initially to spread swap across seperate
> non-raid partitions on 4 of these disks, but the downside of that is if I
> put 2gb on each disk, then I can only swap processes that will fit in 2gb
> swap space. Also, if one of the disks fails, I have to reboot if anything
> was swapped to that drive.
> My questions are as follows:
> What experience are others having with putting swap space on raid
> partitions? I was thinking about maybe swapping on a raid10 device,
> otherwise an LVM spanning multiple drives.
> In practice, what kinds of swap allocation are people finding useful for a
> kvm virtual host of this size?
> I definitely don't want a system that is so overcommited that performance is
> impacted, but if some overcommitment is reasonable for VM's that have light
> workload, then I consider that. I can increase system resources when that
> becomes necessary.
I recommend against using any type of parity-based RAID (5 or 6) for
any workload, period. RAID10 is the only way to go for everything
these days. VMs often run into IO issues, and RAID is going to
make that problem worse. Also, RAID10 in software shouldn't have much
performance impact at all, as opposed to RAID which will have a
big load in software.
I wouldn't worry as much about swap, as others have said. Money spent
adding another device for swap would be better spent on more RAM.
However, I usually put swap on the same device as everything else. It
might not strictly be best practice, but again, if you're hitting
swap, you have bigger problems.
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