Re: [Qemu-devel] QEMU detachable overlays and Union Mounts

[Richard W.M. Jones]

On Wed, Jan 08, 2014 at 11:26:28AM +0530, kausik pal wrote:
> If this message should go elsewhere, my apologies.
> 
> It would be great if QEMU have the capability of detachable overlay or
> union mount.
> So administrators can keep each software as separate individual qcow2/qed
> disks and can put those disks on different layers so that the end user
> would see combination of the softwares available to them.
> 
> The desired scenario is like the following:-

[easier to see the diagrams here:
https://lists.gnu.org/archive/html/qemu-devel/2014-01/msg00819.html]

qemu already supports snapshots.  However they don't work like docker,
ie. you cannot arbitrarily compose snapshots (I don't think this works
properly in docker either, but that's another matter).

Basically because snapshots are block-based, not file based, it is
never going to be possible to compose them in the way you want.
Filesystems at the block layer simply don't work this way.

Have you considered using 9p? [http://wiki.qemu.org/Documentation/9psetup]

Rich.

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