Comment 11 for bug 568445

I noticed the high iowait times a few weeks back when my guest backups were taking a long time to complete. I believe this was sometime after I added a VM to serve as a transparent proxy for my network, but can't be entirely certain. Looking at the e-mail'd cron output, it was fairly obvious that the disk I/O was the problem as several of the guest backups were dropping to 2-3MB/sec reported throughput. These backups are started during the night when there is little to no actual activity on the machines. Checking the host's load and cpu usage confirmed that the problem appeared to be disk I/O related. Searching online seemed to indicate similar problems, but they seemed to be with the disk scheduler being cfq and the recommendation was to move to the deadline scheduler, which the system was already using as its default scheduler.

After changing each of the guest's LVM backed drive to cache='none' the backups are completing in much more reasonable time. Average throughput for the backups remained at 10MB/sec or better, host load remained low even during more intensive operations.