[libvirt-users] libvirt support for qcow2 rebase?
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
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
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)
-
Eric Blake -
Kashyap Chamarthy -
Skardal, Harald