[libvirt-users] Questions and a blockpull issue

Eric Blake eblake at redhat.com
Mon Oct 22 19:51:47 UTC 2012

On 10/22/2012 01:18 PM, Abbas wrote:
> Thanks. That does make sense. I was trying to pull from the backing
> image but wanted to flat down snapshot image and backing image. Not
> specifying --base exactly does what I want.

Good to hear.  Remember, with blockpull, --base is the point in the
chain that becomes the new backing image of the active layer, omitting
--base says no backing image at all.

> Two more quick questions though.
> 1. "virsh blockcommit" would be opposite to blockpull, right? But sing
> it with latest stable kvm 1.2.0 tells that this feature is not yet
> supported. Is it still being developed?

The qemu side is in qemu.git, and will be released in December as part
of qemu 1.3.  The libvirt side is in libvirt.git, and will be released
in November as part of libvirt 1.0.0.  But you are correct that there
are no released versions that support blockcommit yet.

> 2. I assume that there is no need to quiesce the image while creating a
> snapshot or after a blockpull since the pull function works at block
> level to avoid fs corruption. Please correct me if I am wrong.

For snapshots, that all depends on what you want to do with the snapshot
- if you want to ever revert to the snapshot (and boot from that state),
then quiescing makes sense; but if snapshots is only an intermediate
step as part of a larger sequence of operations and you will then remove
the intermediate files via blockpull and/or blockcommit, then quiescing
is not necessary.

> I am
> gonna be testing with a few kvm domains running database servers which
> usually are highly vulnerable to db corruption issues if image files are
> manhandled while data is being accessed.

Thankfully, live snapshot, live block pull, and live block commit have
all been designed to avoid image corruption while qemu continues to run.

> The Fedora wiki currently shows the reverse argument order.
> http://fedoraproject.org/wiki/Features/Virt_Live_Snapshots

Thanks for the pointer; I'm updating that page now.

Eric Blake   eblake at redhat.com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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