8:35 a.m.
I have not found support in libvirt (nor virsh) for doing the equivalent
of "qemu-img rebase ....".
The use case:
You have copied a qcow2 stack and the new files have different names or
reside in a different directory. Therefore you need to change the
backing file.
Is there a way to do this? Is this a planned addition to libvirt?
Harald
10:01 a.m.
On 04/17/2013 07:35 AM, Skardal, Harald wrote:
I have not found support in libvirt (nor virsh) for doing the
equivalent
of "qemu-img rebase ....".
Correct - offline image manipulation is something I'd love to see added,
but last time I worked on the issue, I realized that there is a LOT of
design work still needed. With online image chain manipulation, we can
start a long-running job, and qemu keeps track of it for us. So we can
restart libvirtd, and then ask qemu if the job is done yet, thanks to
qemu supporting a Unix socket monitor connection that we can reconnect
to at will. But with offline image manipulation, a long-running job
means spawning a qemu-img child process, and that does not have an
ability to reconnect. So libvirt itself would be responsible for
setting up enough management resources that the management of qemu-img
can survive a libvirtd restart. Not as trivial as I'd like.
The use case:
You have copied a qcow2 stack and the new files have different names or
reside in a different directory. Therefore you need to change the
backing file.
Is there a way to do this? Is this a planned addition to libvirt?
Yes, it's planned, but only when someone has enough time to make it
their highest priority effort in patching efforts. In the meantime, you
have to settle with doing qemu-img calls yourself, then updating
libvirt's state to match your manual changes.
--
Eric Blake eblake redhat com +1-919-301-3266
Libvirt virtualization library http://libvirt.org
12:42 p.m.
On 04/17/2013 07:05 PM, Skardal, Harald wrote:
I have not found support in libvirt (nor virsh) for doing the
equivalent of “qemu-img
rebase ….”.
The use case:
You have copied a qcow2 stack and the new files have different names or reside in a
different directory. Therefore you need to change the backing file.
Is there a way to do this?
Um, I don't see it yet in libvirt.
But I recently did a small test for which does a diff of images using qemu-img rebase
(this was originally discussed documented by Rich Jones in qemu-img man page):
Posting the info here for convenience:
#----------------------------------------------------------------#
Use-case for 'diff' of images: Useful for converting a monolithic image back into
a thin
image on top of a common base. (useful when you have copied or cloned a guest, and you
want to get back to a thin image on top of a template or base image)
NOTE: (1) the base-image is base-f17.img ; .
#----------------------------------------------------------------#
[root@localhost difftest]# qemu-img info ../base-f17.qcow2
image: ../base-f17.qcow2
file format: qcow2
virtual size: 5.0G (5368709120 bytes)
disk size: 5.0G
cluster_size: 65536
[root@localhost difftest]#
#----------------------------------------------------------------#
[root@localhost difftest]# rsync -avh --progress ../base-f17.qcow2 modified-f17.qcow2
[root@localhost difftest]# ls
modified-f17.qcow2
#----------------------------------------------------------------#
The modified-f17.qcow2 has 3 extra textfiles, and a directory /export (that's the
'difference' between base-f17.img)
#----------------------------------------------------------------#
[root@localhost difftest]# qemu-img info modified-f17.qcow2
image: modified-f17.qcow2
file format: qcow2
virtual size: 5.0G (5368709120 bytes)
disk size: 5.0G
cluster_size: 65536
#----------------------------------------------------------------#
[root@localhost difftest]# qemu-img create -f qcow2 -b modified-f17.qcow2 diff.qcow2
Formatting 'diff.qcow2', fmt=qcow2 size=5368709120
backing_file='modified-f17.qcow2'
encryption=off cluster_size=65536 lazy_refcounts=off
#----------------------------------------------------------------#
[root@localhost difftest]# qemu-img info diff.qcow2
image: diff.qcow2
file format: qcow2
virtual size: 5.0G (5368709120 bytes)
disk size: 196K
cluster_size: 65536
backing file: modified-f17.qcow2
[root@localhost difftest]#
#----------------------------------------------------------------#
Now, let's do the 'rebase' operation. (this is an expensive operation)
- At the end of operation, the diff.qcow2 will have its backing file changed to
'base-f17.qcow2'
- Note that: (In 'Safe mode', which is the default -- any clusters that differ
between
backing_file and the old backing file of filename are merged into filename before
actually
changing the backing file. )
#----------------------------------------------------------------#
[root@localhost difftest]# qemu-img rebase -b ../base-f17.qcow2 diff.qcow2
#----------------------------------------------------------------#
[root@localhost difftest]# qemu-img info diff.qcow2
image: diff.qcow2
file format: qcow2
virtual size: 5.0G (5368709120 bytes)
disk size: 1.4M
cluster_size: 65536
backing file: ../base-f17.qcow2
[root@localhost difftest]#
#----------------------------------------------------------------#
guestfish is able to boot into the diff, and also list the files added here (at this
point, we can discard 'modified.qcow2
#----------------------------------------------------------------#
[root@localhost difftest]# guestfish --rw -i -a diff.qcow2
Welcome to guestfish, the libguestfs filesystem interactive shell for
editing virtual machine filesystems.
Type: 'help' for help on commands
'man' to read the manual
'quit' to quit the shell
Operating system: Fedora release 17 (Beefy Miracle)
/dev/mapper/vg_none-lv_root mounted on /
/dev/sda1 mounted on /boot
<fs> ls /export
testfile1
testfile2
testfile3
<fs>
#----------------------------------------------------------------#
Now, the overlay is quite small, as it can be noticed here:
#----------------------------------------------------------------#
[root@localhost difftest]# ls -lash diff.qcow2
1.4M -rw-r--r--. 1 root root 1.5M Oct 4 05:58 diff.qcow2
[root@localhost difftest]#
#----------------------------------------------------------------#
Also, you can quickly boot into the diff.qcow2 to see what's up with it.
#----------------------------------------------------------------#
[root@localhost difftest]# qemu-kvm -enable-kvm -m 1024 diff.qcow2 -nographic
#----------------------------------------------------------------#
Also posted here - http://kashyapc.fedorapeople.org/virt/img-diff-test.txt
Hope that somewhat answers your question.
Is this a planned addition to libvirt?
Harald
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participants (3)
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Eric Blake -
Kashyap Chamarthy -
Skardal, Harald