Hmmmm. I have a new theory about why my Yosemite Mac is so damn slow.
I turned it off and (without any scientific tests) seemed to get a much better experience. I disabled it by adding the hard drive in the exclusions of Spotlight preferences.
Of course, I immediately started getting complaints from “other users” that they couldn’t find anything so I’ve had to be a bit less harsh on the exclusions.
I also wonder why indexing activity is not visible using
iotop. Maybe it is and I’m just not looking at the output correctly or maybe it happens at such a low level that it doesn’t trigger any counters. Or maybe it all happens in memory in kernel interaction with the file buffer cache (unlikely but possible). However, if that were the case I would still expect kernel cpu to be logged.
Certainly worthy of a bit more investigation, i.e. “googling” plus some dtrace perhaps.