Re: [fedora-virt] images on tmpfs?

On Sat, Jun 07, 2014 at 11:22:10AM -0400, Kanwar Ranbir Sandhu wrote:
> On Sat, 2014-06-07 at 12:33 +0100, Richard W.M. Jones wrote:
> > In any case, cache='none' (ie. O_DIRECT) won't work, and you shouldn't
> > use it for throwaway test machines anyway. Since tmpfs always
> > disappears at reboot, any machine you create on a tmpfs is a throwaway
> > one, whether you intended that or not!
> >
> > Use cache='unsafe' on tmpfs.
> >
> > (This advice applies to libguestfs, virt-builder, etc too)
> >
> > For more info on caching modes, see:
> > http://rwmj.wordpress.com/2013/09/02/new-in-libguestfs-allow-cache-mode-to-be-selected/
>
>
> Does the type of storage being used for the KVM (e.g. LVM, a file, etc.)
> change which cache mode should be employed?

There's no hard rule. You need to test realistic workloads to find
out which is best on your hardware.

> I ask because this page
> says if raw volumes or partitions are used, the cache should be set to
> "none":
>
> http://www.linux-kvm.org/page/Tuning_KVM
>
> Does that still hold true?

There is one pretty big reason to use cache=none, even though its
performance is fairly terrible: Live migration is not possible unless
you use cache='none'.

http://wiki.qemu.org/Migration/Storage

Rich.

--
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