Re: [Qemu-devel] Re: Strategic decision: COW format

[Markus Armbruster]

Chunqiang Tang <address@hidden> writes:

[...]
> Now let’s talk about features. It seems that there is great interest in 
> QCOW2’ internal snapshot feature. If we really want to do that, the right 

Great interest?  Its use cases are demo, debugging, testing and such.
Kind of useful for developers, but I wouldn't want to use it in anger.
Nice to have if we can get it cheaply, but I'm not prepared to pay much
for it in performance or complexity, and I doubt I'm the only one.

Users always say "yes" when you ask them whether they need some feature.
Hence, the question is useless.  A better question to ask is "how much
are you willing to pay for it?"

> solution is to follow VMDK’s approach of storing each snapshot as a 
> separate COW file (see http://www.vmware.com/app/vmdk/?src=vmdk ), rather 
> than using the reference count table. VMDK’s approach can be easily 
> implemented for any COW format, or even as a function of the generic block 
> layer, without complicating any COW format or hurting its performance. I 
> know the snapshots are not really “internal” as stored in a single file 
> but instead more like external snapshots, but users don’t care about that 
> so long as they support the same use cases. Probably many people who use 
> VMware don't even know that the snapshots are stored as separate files. Do 
> they care?

I certainly wouldn't.