qcow2 snapshotting: does it influence future IO performance?

[jurgen.depicker at let.be]

Hi Alvin.

I read on your site that you stopped work on a project because LVM 
snapshots potentially decreased IO performance, and I also saw that you 
worked on a perl project to make copies of VMs, so therefore I address my 
question directly to you.

Your work reminded me of something I read some weeks ago on qcow2 on 
http://people.gnome.org/~markmc/qcow-image-format.html and which raised my 
brows at the time:
"Snapshots - "real snapshots" - are represented in the original image 
itself. Each snapshot is a read-only record of the image a past instant. 
The original image remains writable and as modifications are made to it, a 
copy of the original data is made for any snapshots referring to it. "

I tried to find more info about this, but didn't yet.  So here's my 
question: does anybody know whether, having made snapshots of a VM during 
several stages of its life (clean install, important service installed, 
...), affects IO performance of any writes made after snapshotting, as one 
could suspect from my quote above?

Regards,
Jürgen
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