[Qemu-devel] snapshot vs COW?

[Qemu-devel] snapshot vs COW?

[Jun Koi]

Hi,

>From what I see, both snapshot and COW features can be used to try
some changes on the file image before we the change back to the
original image.

But what is the advantages of snapshot over COW, and in which case we
should use snapshot, but not COW? Vice verse, when COW is more
appropriate?

Many thanks,
Jun

[Qemu-devel] Re: snapshot vs COW?

[Jun Koi]

On Fri, Dec 12, 2008 at 12:17 PM, Jun Koi <junkoi2004@gmail.com> wrote:
> Hi,
>
> From what I see, both snapshot and COW features can be used to try
> some changes on the file image before we the change back to the
> original image.

I mean "... before we merge then changes back ..."

Thanks,
Jun

[Qemu-devel] Re: snapshot vs COW?

[walt]

Jun Koi wrote:
> Hi,
>
>  From what I see, both snapshot and COW features can be used to try
> some changes on the file image before we the change back to the
> original image.
>
> But what is the advantages of snapshot over COW, and in which case we
> should use snapshot, but not COW? Vice verse, when COW is more
> appropriate?
>
> Many thanks,
> Jun

COW is a nice feature but it is currently broken, so don't use it
until it gets fixed.  When you commit any changes your backing file
will be corrupted.

Re: [Qemu-devel] Re: snapshot vs COW?

[Jamie Lokier]

walt wrote:
> Jun Koi wrote:
> >Hi,
> >
> > From what I see, both snapshot and COW features can be used to try
> >some changes on the file image before we the change back to the
> >original image.
> >
> >But what is the advantages of snapshot over COW, and in which case we
> >should use snapshot, but not COW? Vice verse, when COW is more
> >appropriate?
> >
> >Many thanks,
> >Jun
> 
> COW is a nice feature but it is currently broken, so don't use it
> until it gets fixed.  When you commit any changes your backing file
> will be corrupted.

Aieee!  I use COW instead of snapshots in my management scripts,
because snapshots are harder to manage.

Is the COW brokenness documented?  What version did it start being broken?

-- Jamie

Re: [Qemu-devel] Re: snapshot vs COW?

[Thiemo Seufer]

Jamie Lokier wrote:
> walt wrote:
> > Jun Koi wrote:
> > >Hi,
> > >
> > > From what I see, both snapshot and COW features can be used to try
> > >some changes on the file image before we the change back to the
> > >original image.
> > >
> > >But what is the advantages of snapshot over COW, and in which case we
> > >should use snapshot, but not COW? Vice verse, when COW is more
> > >appropriate?
> > >
> > >Many thanks,
> > >Jun
> > 
> > COW is a nice feature but it is currently broken, so don't use it
> > until it gets fixed.  When you commit any changes your backing file
> > will be corrupted.
> 
> Aieee!  I use COW instead of snapshots in my management scripts,
> because snapshots are harder to manage.
> 
> Is the COW brokenness documented?  What version did it start being broken?

Cow2 is broken in the 0.9.1 release, it was fixed shortly afterwards.
The symptom was corruption of the image when it grew beyond 4 GB.

AFAIK you can still break cow2 images by running out of disk space.
This is the case since cow2 suport was written.


Thiemo

[Qemu-devel] Re: snapshot vs COW?

[walt]

Thiemo Seufer wrote:
> Jamie Lokier wrote:
>> walt wrote:
>>> COW is a nice feature but it is currently broken, so don't use it
>>> until it gets fixed.  When you commit any changes your backing file
>>> will be corrupted.

>> Aieee!  I use COW instead of snapshots in my management scripts,
>> because snapshots are harder to manage.
>>
>> Is the COW brokenness documented?  What version did it start being broken?

> Cow2 is broken in the 0.9.1 release, it was fixed shortly afterwards.
> The symptom was corruption of the image when it grew beyond 4 GB...

Glad to hear it, thanks for the update.  That means that the fix is
already in the kvm git repository?