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

[Alex Bligh]

On 14 Aug 2013, at 14:43, Kaveh Razavi wrote:

> No, once the read-only cache is created, it can be used by different VMs
> on the same host. But yes, it first needs to be created.

OK - this was the point I had missed.

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'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.

-- 
Alex Bligh