Re: [Qemu-devel] [PATCH] Introduce cache images for the QCOW2 format

[Kaveh Razavi <kaveh@cs.vu.nl>]

On 08/14/2013 03:50 PM, Alex Bligh wrote:
> Assuming the cache quota is not exhausted, how do you know how that
> a VM has finished 'creating' the cache? At any point it might
> read a bit more from the backing image.

I was assuming on shutdown.

> I'm wondering whether you could just use POSIX mandatory locking for
> this, i.e. open it exclusive and r/w until the 'finish point', then
> reopen RO, which would allow other VMs to share it. Any other VMs
> starting before the cache was populated simply fail to get the
> exclusive lock and go direct to the backing file.

This is a good idea, since it relaxes the requirement for releasing the
cache only on shutdown. I am not sure how the 'finish point' can be
recognized. Full cache quota is one obvious scenario, but I imagine most
VMs do/should not really read till that point (unless they are doing
something that should not be cached anyway - e.g. file-system check).
Another possibility is registering some sort of event to be executed
periodically (e.g. every 30 seconds), and if the cache is not modified
in a period, then that is the 'finish point'. I do not know how feasible
that is with the facilities that qemu provides.

Kaveh