Finally, after I don’t know HOW many years, I figured out how to get continuously updated stats from iostat that don’t just scroll up the screen and piss you off.
For those of you who aren’t familiar, iostat gives you some really awesome per-disk reports that you can use to look for problems. Eg, on a system I’m moving a bunch of data around on at the moment:
root@dr0:~# iostat --human -xs
Linux 4.15.0-45-generic (dr0) 06/04/2019 x86_64 (16 CPU)
avg-cpu: %user %nice %system %iowait %steal %idle
0.1% 0.0% 2.6% 5.8% 0.0% 91.5%
Device tps kB/s rqm/s await aqu-sz areq-sz %util
loop0 0.00 0.0k 0.00 0.00 0.00 1.6k 0.0%
sda 214.70 17.6M 0.14 2.25 0.48 83.9k 29.8%
sdb 364.41 38.1M 0.63 9.61 3.50 107.0k 76.5%
sdc 236.49 22.2M 4.13 2.13 0.50 96.3k 20.4%
sdd 237.14 22.2M 4.09 2.14 0.51 95.9k 20.4%
md0 12.09 221.5k 0.00 0.00 0.00 18.3k 0.0%
In particular, note that %util column. That lets me see that /dev/sdb is the bottleneck on my current copy operation. (I expect this, since it’s a single disk reading small blocks and writing large blocks to a two-vdev pool, but if this were one big pool, it would be an indication of problems with sdb.)
But what if I want to see a continuously updated feed? Well, I can do iostat –human -xs 1 and get a new listing every second… but it just scrolls up the screen, too fast to read. Yuck.
OK, how about using the watch command instead? Well, normally, when you call iostat, the first output is a reading that averages the stats for all devices since the first boot. This one won’t change visibly very often unless the system was JUST booted, and almost certainly isn’t what you want. It also frustrates the heck out of any attempt to simply use watch.
The key here is the -y argument, which skips that first report which always gives you the summary of history since last boot, and gets straight to the continuous interval reports – and knowing that you need to specify an interval, and a count for iostat output. If you get all that right, you can finally use watch -n 1 to get a running output of iostat that doesn’t scroll up off the screen and drive you insane trying to follow it:
root@dr0:~# watch -n 1 iostat -xy --human 1 1