[Users] mount ploop image from read-only fs

[Roman Haefeli]

On Fre, 2014-10-24 at 15:46 -0700, Kir Kolyshkin wrote:
> Sorry for hijacking the thread,

It's good that you ask this.

>  but back to the original problem.
> Can you tell why vzctl snapshot-mount (or ploop snapshot-mount)
> is/was not working for you? Ideally, please provide a detailed scenario.

Because my problem is not related to ploop snapshots. Our ploop images
are lying on an NFS storage, which is exported to and mounted by OpenVZ
hostnodes. The shared filesystem is snapshotted nightly and snapshots
are available to the hostnodes read-only. 'ploop mount' doesn't seem to
work when the ploop images are on a ro-filesystem (I figured because it
wants to set the dirty flag, is that correct?). 'vzctl snapshot-mount'
and 'ploop snapshot-mount' don't help because many of our CTs don't have
any ploop snapshots.

Ideally, 'ploop mount' would have some flag to allow for read-only
mounts without touching the ploop-image at all. I say that without
knowing the architectural implications of this, but from a user
perspective it would be useful.

Then I learned from Pavel that there is a userspace utility doing what I
want. 

 
Roman