PostgreSQL 10 High Performance - Third Edition

Disk performance expectations

So what are the reasonable expectations for how your disks should perform? The last example shown demonstrates how things should work. Any good drive nowadays should have sequential transfers of well over 50 Mbps on its fastest area, with 100 Mbps being easy to find. The slowest part of the drive will be closer to half that speed. It's good practice to try and test an individual drive before building more complicated arrays using them. If a single drive is slow, you can be sure an array of them will be bad too.

The tricky part of estimating how fast your system should be is when you put multiple drives into an array.

For multiple disks in a RAID 1 array, the sequential read and write speed will not increase. ...