When busy but not overloaded, iostat data for this system looks like the following:
$ iostat 5
avg-cpu: %user %nice %system %iowait %steal %idle
18.54 0.00 9.45 23.49 0.15 48.38
Device tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn
sdc1 1068.80 0.00 15740.80 0 78704
sdd1 1041.80 0.00 15459.20 0 77296
sde1 1028.00 0.00 15377.60 0 76888
md0 5969.20 0.00 47753.60 0 238768
sdf1 989.00 0.00 40449.60 0 202248
The %iowait figure of 23% is high enough to know the disks are busy, but not completely saturated yet. This is showing 20 Mbps (40449.6 512-byte blocks per second) being written to the WAL and 24 Mbps to the entire database disk array, the latter of which is evenly split as almost 8 Mbps to each of the three drives. ...
