[libvirt] qemu and <transient/> disks

[Richard W.M. Jones]

On Mon, Jul 23, 2012 at 10:28:57AM +0100, Daniel P. Berrange wrote:
> I agree with the quoted text that libvirt should define a
> directory to use for the transient disks under /var/lib/libvirt/images
> perhaps, and just use 'qemu-img create' to create that, and the
> unlink it once QEMU has started up, so we can auto-cleanup
> on QEMU shutdown.

One issue here is that 'qemu-img create' is slow.  0.4 seconds to
create a qcow2 file backing another file (ie. per readonly disk, and
programs like virt-df may open dozens of readonly disks).

I looked at the strace output and it seems the slowness is just
overhead from the machinery of qemu's coroutines, and not connected
with writing the qcow2 file, which is presumably why the qemu
snapshot=on option is not noticably slow.

Rich.

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