I noticed that I had an field in top labeled "st" and that it was busy once I switched some Xen domains to be file backed (tap:aio) instead of LVM backed (phy). Anyway, after doing some investigation I found that "st" means "steal_time" and is "the cpu time spent in involuntary wait." This is time spent executing other virtual domains in Xen or other virtualization applications. I couldn't find the details when searching for information about the top command (top being one of those difficult search terms). But since top gets its values from /proc/stat I was able to use that to aid my research. I ended up finding the details at http://www.linuxhowtos.org/System/procstat.htm and http://kerneltrap.org/node/8235.