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


From: Kaveh Razavi
Subject: Re: [Qemu-devel] [PATCH] Introduce cache images for the QCOW2 format
Date: Wed, 14 Aug 2013 16:26:05 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130804 Thunderbird/17.0.8

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



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