[fedora-virt] QCOW2 vs QROW2 Disks

[Kashyap Chamarthy]

On 10/28/2012 02:07 AM, Jorge Fábregas wrote:
> Hi there,
> 
> I'm just curious:  Where does the "Copy on Write" (from QCOW) comes
> from?  Everything I've been reading regarding how snapshots work with
> QEMU points to a "redirect on write" approach.  Or is the COW part
> related to some other internal mechanism (not related to snapshots)?

The COW part /is/ related to  external disk snapshots:

Whenever you take external disk-only snapshot, the current disk image in use is marked
read-only(while serving as a backing file for the rest of the snapshots taken), and a new
QCOW2 overlay is created to track the new 'writes'(or delta) from there on.

It's worth noting that, the base image can be RAW or QCOW2, but the snapshots(or overlays)
are always QCOW2.

(Note that - read-only doesn't mean, file-system permissions, but 'qemu' opens the disk
read-only.)


See if this are helpful :

http://kashyapc.wordpress.com/2012/09/14/externaland-live-snapshots-with-libvirt/

http://kashyapc.wordpress.com/2012/10/04/creating-rapid-thin-provisioned-guests-using-qemu-backing-files/

/kashyap

> 
> Thanks in advance,
> Jorge
> _______________________________________________
> virt mailing list
> virt at lists.fedoraproject.org
> https://admin.fedoraproject.org/mailman/listinfo/virt
>