On 05/12/16 01:33, Francois Gouget wrote:
On Sun, 4 Dec 2016, Jonas Maebe wrote:
Francois Gouget wrote:
Indeed during the revert vm1 shows no read traffic (lots of RAM to cache that), but a steady 6 MB/s stream of writes ! In contrast on vm2 writes quickly ramp up to 80 MB/s, then stop after ~5 seconds and QEMU just uses CPU for the last ~4 seconds.
Maybe vm2 mounts its file systems with noatime?
The VM hosts all use relatime which provides essentially all the benefits of noatime.
I actually meant the file system in the VM, but I guess there are Windows rather than Linux VMs? Additionally, what is the "revert" operation exactly? Is it like an "svn revert"/"git reset --hard HEAD" in the VM, or some qemu operation, or something else?
But in this case I expect the writes all go to the qcow2 disk image and I know vm1 is capable of sustaining more than 6 MB/s writes (e.g. when copying >100 GB around).
One thing you could look at is the output of iostat on the host while the operations are going on, in partical the transactions-per-second, to check whether the issue is that one is using a lot of small writes (for what ever reason) while the other uses fewer, larger writes.
Jonas