Re: [Qemu-devel] [PATCH 0/2] Update documentation for qemu-img + add new -B option

["Marc Bevand" <m.bevand@gmail.com>]

On Wed, Jun 4, 2008 at 8:39 AM, Jamie Lokier <jamie@shareable.org> wrote:
>
>  - When using a relative image, you should be able to set the
>    permissions on the base image to read-only.  If you have a lot of
>    relative images, it's really important not to modify the base
>    image, and worrying that one of your QEMU processes might write to
>    the base image.  Virtual PC allows the base image to be read-only,
>    and quite a lot of notes on Differencing stress how important it
>    is to set permissions that way, if you are sharing the base.

I have been using qemu this way for a while: hundreds of cow images
based on a small set of read-only base images (that were also
chattr +i to prevent accidents).

>  - I'm never sure from the documentation what happens when using
>    '-snapshot' with a relative image, and you issue the 'commit'
>    monitor command.  Does it commit the snapshot to the relative
>    image, or the snapshot all the way to the base image (as help suggests).

I am pretty sure it commits to the relative image though I have
never used -snapshot and have never looked at the code.

>  - Same for images which are relative to other relative images,
>    i.e. nested.  How far up the tree does 'commit' write too?

To the first base image. The commit command always commit
to the next base image only in the chain. If you have A, based
on B, based on C, the only way to commit to C is to open B and
issue "commit".

>  - qemu's 'vpc' format support isn't mentioned in the man page.
>    That would explain why someone submitted another implementation a
>    while back.

Yeah the doc is generally vastly outdated... There are tons of
undocumented qemu options too.

-- 
Marc Bevand