Re: [Qemu-devel] Regression opening read-only cdroms

[Jamie Lokier <jamie@shareable.org>]

Avi Kivity wrote:
> On 06/16/2009 05:33 PM, Jamie Lokier wrote:
> >Avi Kivity wrote:
> >   
> >>qemu used to be quite happy opening read-only cdrom images, and I was
> >>quite happy feeding kvm-autotest a library of read-only iso images.
> >>     
> >
> >1. While we're here, an _option_ to open an image read-only even when
> >    you have write permission would be useful, for those occasions when
> >    you want to boot from some valuable image and be certain you aren't
> >    modifying it - without having to chmod back and forth in
> >    Qemu-wrapper scripts, or copy the image first.
> >   
> 
> read-only disk images don't make much sense.

And yet "chmod 444 image; qemu ..." works.
If you're booting from a disk you don't need to write to, obviously.
Generally it'll need to be mounted read-only in the guest.

> Using -snapshot will generally ensure the image is not modified, while 
> allowing the guest to write.

I never do that with _valuable_ images because:

   - Valuable images are expensive/difficult/impossible to recreate.
     But too large to copy about casually.

   - I don't have that much faith in QEMU's correctness, having
     already been bitten by a number it's bugs, or in the guest's
     correctness if I were to rely on the guest doing read-only mount
     instead of using -snapshot.

   - It's too easy to accidentally write back the changes over the
     the original image.  Man page:

     "the raw disk image you use is not written back. You can however
     force the write back by pressing C-a s".

And I don't do it when booting a guest where I have _both_ a disk a do
want to write, and another valuable image that I don't want written,
because:

   - How would I use -snapshot and then commit changes to the disk I
     do want to write (either C-a s or "commit" in the monitor), but
     never write changes to the disk I don't want written?

   - "commit" has always been a bit ambiguous when applied to a
     combination of -snapshot and a qcow2 delta image.

Finally, QEMU clearly does support read-only images, so it's always
struck me as odd that the only way to invoke this support is using
"chmod" outside QEMU.  (Actually I use "chattr +i" as well.  That's
how paranoid I am about difficult to recreate images).

-- Jamie