I'm trying to debug some weird behaviour with Postgres performance and am trying to understand the iostat output. I'm on 2x SSD on RAID1. Why would one of the disks be reported at ~21% utilisation and the other at ~95% utilisation? Shouldn't they be exactly the same?
Could this in any way be related to this question?
avg-cpu: %user %nice %system %iowait %steal %idle 10.03 0.00 5.28 1.24 0.00 83.45 Device r/s w/s rMB/s wMB/s rrqm/s wrqm/s %rrqm %wrqm r_await w_await aqu-sz rareq-sz wareq-sz svctm %util loop0 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 sda 83.20 493.60 0.72 8.11 0.00 1.80 0.00 0.36 0.56 0.39 0.24 8.83 16.83 0.37 21.36 sdb 24.40 493.60 0.24 8.11 0.00 1.80 0.00 0.36 3.41 1.91 1.02 10.10 16.83 1.82 94.24 md0 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 md1 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 md2 107.60 494.60 0.96 8.12 0.00 0.00 0.00 0.00 0.00 0.00 0.00 9.12 16.81 0.00 0.00 
%utilthe manualman iostatgives a big caveat with regards to SSD's: "for devices serving requests in parallel, such as RAID arrays and modern SSDs, this number does not reflect their performance limits.`" - so by itself it is hard to say if those different numbers are significant or not. Maybe take a look at the SMART health status of the drives?smartctl -xall /dev/sdaresp. /dev/sdb