[CentOS] Swap space for kvm virtual host

Brian Mathis 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.
> Nataraj

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[56] is going to
make that problem worse.  Also, RAID10 in software shouldn't have much
performance impact at all, as opposed to RAID[56] 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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