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

[Alex Bligh]

On 14 Aug 2013, at 15:26, Kaveh Razavi wrote:

> 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.

You can set timers and modify them - that's easy enough. I suspect the
heuristic may need some tuning, but a simple one such as '60 seconds
after boot' might be sufficient. After all, if a VM takes more than
60 seconds to boot and does not fill the cache quota in that time,
I think we can assume the cache isn't going to do it much good!

-- 
Alex Bligh