Saturday, June 7, 2014

Re: [fedora-virt] images on tmpfs?

On Thu, Jun 05, 2014 at 12:54:19PM -0400, Tom Horsley wrote:
> On Thu, 05 Jun 2014 10:30:31 -0600
> Eric Blake wrote:
> > What caching mode are you requesting? tmpfs can't (yet) support
> > O_DIRECT, and if the caching mode you request causes qemu to try
> > O_DIRECT, that would explain why it is failing.
> Is that qemu's default mode? I don't think anyone is explicitly
> setting it, but I'll have to find out from the folks actually
> trying this.

I can't recall what the defaults are, but if you run 'virsh dumpxml'
it will tell you the cache mode for each drive.

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:


Richard Jones, Virtualization Group, Red Hat
Read my programming and virtualization blog:
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