Hello,

Since several people asked to post the results, here they are.
I tried raw virtio disk with and without -z -C set and also qcow2 virtio 
disk without -z -C set and did not notice any difference in performance 
at all - Redhat 6.2 Minimal installs in 10 minutes in each case. The 
"abysmal" performance as it was some several months ago (like 10 minutes 
just for virtual disk formatting) under the same conditions is no more 
at least on 3.3.0-rc5.

best
~dima


On 02/24/2012 02:22 PM, dima wrote:

On 02/13/2012 04:17 PM, Ralf-Peter Rohbeck wrote:

quoted

Hello,
is it possible to set nodatacow on a per-file basis? I couldn't find
anything.
If not, wouldn't that be a great feature to get around the performance
issues with VM and database storage? Of course cloning should still
cause COW.

Hello,
Going back to the original question from Ralf I wanted to share my
experience.

Yesterday I set up KVM+qemu and set -z -C with David's 'fileflags'
utility for the VM image file.
I was very pleased with results - Redhat 6 Minimal installation was
installed in 10 minutes whereas it was taking 'forever' the last time I
tried it some 4 months ago. Writes during installation were very
moderate. Performance of VM is excellent. Installing some big packages
with yum inside VM goes very quickly with the speed indistinguishable
from that of bare metal installs.

I am not quite sure should this improvement be attributed to the nocow
and nocompress flags or to the overall improvement of btrfs (I am on
3.3-rc4 kernel) but KVM is definitely more than usable on btrfs now.

I am yet to test the install speed and performance without those flags set.

best
~dima
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