qcow2 snapshotting: does it influence future IO performance?

jurgen.depicker at let.be jurgen.depicker at let.be
Thu Mar 31 13:15:35 UTC 2011

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?

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