libvirt users - Feb 2014 - Re: Can I move the disk image of the guest while it is running?

----- Original Message -----
> From: "Eric Blake" <eblake@redhat.com>
> To: "Andrew Martin" <amartin@xes-inc.com>
> Cc: "Gergely Horváth" <gergely.horvath@inepex.com>,
libvirt-users@redhat.com
> Sent: Thursday, February 6, 2014 9:31:29 AM
> Subject: Re: [libvirt-users] Can I move the disk image of the guest while
it is running?
> 
> On 02/06/2014 08:14 AM, Andrew Martin wrote:
> > Eric,
> > 
> > For scripted live backups, which non-transient technique would you
prefer:
> > 1. snapshot-create-as followed by blockpull, which results in the
original
> > file being left as a standalone copy of the VM from that point in
time; it
> > can then be copied off to a backup disk once blockpull completes
> 
> Slower (because it pulls the entire contents of the original file into
> the new file), but likely to succeed.
Is there a faster, non-transient way to do this with the current versions of
qemu and libvirt?
> 
> > 2. snapshot-create-as followed by copying the backing file (since it
is now
> > read-only) and then using blockcommit to pull the interim changes back
into
> > the original/backing file (and removing the snapshot file)
> 
> Ideal, but not supported until (unreleased) qemu 2.0.
> 
Ah, does blockcommit currently only work with offline VMs with qemu 1.x?

On 02/06/2014 01:25 PM, Andrew Martin wrote:
> ----- Original Message -----
>> From: "Eric Blake" <eblake@redhat.com>
>> To: "Andrew Martin" <amartin@xes-inc.com>
>> Cc: "Gergely Horváth" <gergely.horvath@inepex.com>,
libvirt-users@redhat.com
>> Sent: Thursday, February 6, 2014 9:31:29 AM
>> Subject: Re: [libvirt-users] Can I move the disk image of the guest
while it is running?
>>
>> On 02/06/2014 08:14 AM, Andrew Martin wrote:
>>> Eric,
>>>
>>> For scripted live backups, which non-transient technique would you
prefer:
>>> 1. snapshot-create-as followed by blockpull, which results in the
original
>>> file being left as a standalone copy of the VM from that point in
time; it
>>> can then be copied off to a backup disk once blockpull completes
>>
>> Slower (because it pulls the entire contents of the original file into
>> the new file), but likely to succeed.
> Is there a faster, non-transient way to do this with the current versions
of qemu and libvirt?
Sadly, no.  The whole area of block chain management is still undergoing
active development, and qemu 2.0 definitely adds some nice features to
the mix that libvirt will have to be taught to support.
> 
>>
>>> 2. snapshot-create-as followed by copying the backing file (since
it is now
>>> read-only) and then using blockcommit to pull the interim changes
back into
>>> the original/backing file (and removing the snapshot file)
>>
>> Ideal, but not supported until (unreleased) qemu 2.0.
>>
> Ah, does blockcommit currently only work with offline VMs with qemu 1.x?
Blockcommit does not yet work with offline VMs.  But you can get the
same behavior as what offline blockcommit would do by using qemu-img
manually.  One idea for a patch is to see if 'qemu -M none' can drive a
block commit operation using a long-running qemu process that does
nothing but the block operation; if that works, then we could wire up
libvirt to do long-running block operations that survive a libvirtd
restart.  But the reason libvirt doesn't currently support offline
operations is that we have no way to keep a long-running qemu-img
operation running smoothly across a libvirtd restart.

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

----- Original Message -----
> From: "Eric Blake" <eblake@redhat.com>
> To: "Andrew Martin" <amartin@xes-inc.com>
> Cc: "Gergely Horváth" <gergely.horvath@inepex.com>,
libvirt-users@redhat.com
> Sent: Thursday, February 6, 2014 2:35:35 PM
> Subject: Re: [libvirt-users] Can I move the disk image of the guest while
it is running?
> 
> On 02/06/2014 01:25 PM, Andrew Martin wrote:
> > ----- Original Message -----
> >> From: "Eric Blake" <eblake@redhat.com>
> >> To: "Andrew Martin" <amartin@xes-inc.com>
> >> Cc: "Gergely Horváth"
<gergely.horvath@inepex.com>,
> >> libvirt-users@redhat.com
> >> Sent: Thursday, February 6, 2014 9:31:29 AM
> >> Subject: Re: [libvirt-users] Can I move the disk image of the
guest while
> >> it is running?
> >>
> >> On 02/06/2014 08:14 AM, Andrew Martin wrote:
> >>> Eric,
> >>>
> >>> For scripted live backups, which non-transient technique would
you
> >>> prefer:
> >>> 1. snapshot-create-as followed by blockpull, which results in
the
> >>> original
> >>> file being left as a standalone copy of the VM from that point
in time;
> >>> it
> >>> can then be copied off to a backup disk once blockpull
completes
> >>
> >> Slower (because it pulls the entire contents of the original file
into
> >> the new file), but likely to succeed.
> > Is there a faster, non-transient way to do this with the current
versions
> > of qemu and libvirt?
> 
> Sadly, no.  The whole area of block chain management is still undergoing
> active development, and qemu 2.0 definitely adds some nice features to
> the mix that libvirt will have to be taught to support.
> 
> > 
> >>
> >>> 2. snapshot-create-as followed by copying the backing file
(since it is
> >>> now
> >>> read-only) and then using blockcommit to pull the interim
changes back
> >>> into
> >>> the original/backing file (and removing the snapshot file)
> >>
> >> Ideal, but not supported until (unreleased) qemu 2.0.
> >>
> > Ah, does blockcommit currently only work with offline VMs with qemu
1.x?
> 
> Blockcommit does not yet work with offline VMs.  But you can get the
> same behavior as what offline blockcommit would do by using qemu-img
> manually.  One idea for a patch is to see if 'qemu -M none' can
drive a
> block commit operation using a long-running qemu process that does
> nothing but the block operation; if that works, then we could wire up
> libvirt to do long-running block operations that survive a libvirtd
> restart.  But the reason libvirt doesn't currently support offline
> operations is that we have no way to keep a long-running qemu-img
> operation running smoothly across a libvirtd restart.
Thanks for the clarification!

Andrew