[fedora-virt] QCOW2 vs QROW2 Disks

[Kashyap Chamarthy]

On 10/29/2012 09:54 AM, Kashyap Chamarthy wrote:
> 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:

Now, that I re-read your question (after taking coffee), I didn't answer your question :)

I think, COW means -- on 'write', modify the 'copy'(which is the overlay).

/kashyap

> 
> 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
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> 
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