openstack team mailing list archive

[Gaurab Basu <basugaurab88@xxxxxxxxx>]

Hi Michael,

Thanks for your reply.

Another thing I would like to know is whether it uses snapshot mechanism
over time.
I mean how does the copy on write functionality works. Does it keep the
diff snapshots over time ( or something else ).
And does the diff work at the level of file or block level?

What is the format that the image is converted to after it is fetched from
glance.

I am fairly new to openstack.
Can you point me to the specific files in the code where all these things
are coded. I want to know the details of the
present state.

Thanks again for your help.

Regards,
Gaurab

On Sat, Jul 28, 2012 at 11:52 AM, Michael Still <michael.still@xxxxxxxxxxxxx
> wrote:

> On 28/07/12 05:42, Gaurab Basu wrote:
> > Hi,
> >
> > I am trying to figure out the technology that openstack uses when
> > multiple VM's having the*same *base image (OS) are provisioned on a
> > physical server.
> > Does it use as many copy as the number of VM's or does it use the same
> > base image and then copy on write.
> >
> > I need to understand the complete details. Can anybody share some
> > details or point me to some place where I can find the details.
>
> Its pretty hard to provide a complete description of what happens,
> because the code keeps changing. However, assuming you have copy on
> write turned on (which is the default IIRC), and assuming that all of
> the instances have the same disk size, then you end up with:
>
>  - the image as fetched from glance, with possible format conversion
>  - that image resized to the size the instance requested
>  - a cow on write layer for each instance that is using that sized image
>
> The first should be smallish, the second can be quite large, and the
> third will really depend on how much writing the instances are doing.
>
> Note that this all falls apart if instances are migrated, because as
> part of the migration the copy on write layer is transformed into a full
> disk image, which is what is shipped over to the new machine.
>
> Hope this helps,
> Mikal
>
>