Re: [Qemu-devel] qcow2: online snasphots : internal vs external ?

[Alexandre DERUMIER]

Thank Stefan,that's clear now.


Maybe one more question, about qemu snapshot that I don't understand.

I work since many years with snapshot on zfs or netapp,
and on these system like ceph, I can rollback at the time of the snapshot, and 
have a view of when the snapshot was taken.

exemple :
image1 : empty dir /
take a snapshot (snap1)
touch /file1
now rollback to snap1
ls /   ->empty dir, like when snap1 was taken


now,example with qemu:
image1 : empty dir /
take a snapshot: (qemu-img snapshot -c snap1 image1)
touch /file1
now rollback to snap1 (qemu-img snapshot -a snap1 image1)
ls /file1


the behaviour is completly different. Did I miss something ?


----- Mail original -----

De: "Stefan Hajnoczi" <address@hidden>
À: "Alexandre DERUMIER" <address@hidden>
Cc: "Jeff Cody" <address@hidden>, "qemu-devel" <address@hidden>
Envoyé: Samedi 25 Août 2012 16:01:45
Objet: Re: [Qemu-devel] qcow2: online snasphots : internal vs external ?

On Sat, Aug 25, 2012 at 2:25 PM, Alexandre DERUMIER <address@hidden> wrote:
> I'm currently looking to add live snapshot support to proxmox kvm 
> distribution.
>
> Is it possible to use internal snapshots on a running guest running qcow2 
> disk?
> (qemu-img snapshot -c ) ?

No. qemu-img should not be used if the guest is running.

> Or do I need to use external snapshots with qmp blockdev-snapshot-sync ? 
> (Seem more complex to delete old snapshots)

External qcow2/qed snapshots can be created while the guest is running
using snapshot-blockdev-sync, but it is not yet possible to flatten
the chain arbitrarily while the guest is running.

You can use block-stream to populate the top-most image file with data
from its backing image chain. Jeff Cody is working on block-commit
which allows merging down (this is the opposite of block-stream). The
advantage with block-commit is that backing images are often smaller
than the image file, it's therefore more efficient to copy less data
down instead of copying the backing image up into the image file.

Stefan




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Alexandre D e rumier

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