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

[Alex Bligh]

Kaveh,

On 14 Aug 2013, at 12:28, Kaveh Razavi wrote:

> On 08/14/2013 12:53 AM, Alex Bligh wrote:
>> What is this cache keyed on and how is it invalidated? Let's say a
>> 2 VM on node X boot with backing file A. The first populates the cache,
>> and the second utilises the cache. I then stop both VMs, delete
>> the derived disks, and change the contents of the backing file. I then
>> boot a VM using the changed backing file on node X and node Y. I think
>> node Y is going to get the clean backing file. However, how does node
>> X know not to use the cache? Would it not be a good idea to check
>> (at least) the inode number and the mtime of the backing file correspond
>> with values saved in the cache, and if not the same then ignore the
>> cache?
> 
> You could argue the same for normal qcow2. Start from a cow image with a
> backing image, stop the VM. Start another VM, modifying the backing
> image directly. Start the VM again, this time from the cow image, and
> the VM can see stale data in the stored data clusters of the cow image.

That's if the VM retains a qcow2 based on the backing file. I meant reboot
the two VMs with a fresh qcow2 read/write file.

> The idea is once a user registers an image to a cloud middleware, it is
> assigned an image ID. As long as the middleware assigns a cache to the
> backing image with the same ID, there is no possibility to read stale
> data. If it is desired to have some sort of check at the qemu level, it
> should be implemented in the qcow2 directly for all backing files and
> this extension will benefit from it too.

I don't agree. The penalty for a qcow2 suffering a false positive on
a change to a backing file is that the VM can no longer boot. The
penalty for your cache suffering a false positive is that the
VM boots marginally slower. Moreover, it is expected behaviour that
you CAN change a backing file if there are no r/w images based on
it. Your cache changes that assumption.

-- 
Alex Bligh