On 07/23/2012 03:48 AM, Daniel P. Berrange wrote: > On Mon, Jul 23, 2012 at 10:41:55AM +0100, Richard W.M. Jones wrote: >> 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. I've also seen complaints from people that /var/lib/libvirt can be a space-limited directory; we should really consider adding the notion of a per-VM storage pool (defaulting to a libvirt-internal pool at /var/lib/libvirt, but can be specified by the user to live in a different mount point) for anything that needs potentially large amounts of storage (transient disk wrappers, managed save images, and so forth). >> >> 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. > > Other options available could be > > - Enhance QEMU to allow a snapshot directory to be specified > - Make libvirt actually create the empty QCow2 file. The QEMU > people will probably say this is evil, but the knowledge > required to a create an empty qcow2 file pointing to a backing > file is pretty tiny. Especially since libvirt already has the knowledge on how to read a qcow2 file. > > Or we could even do both, using the former for new enough QEMU and > the latter only for older QEMU > > > Daniel > -- Eric Blake eblake@xxxxxxxxxx +1-919-301-3266 Libvirt virtualization library http://libvirt.org
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