Warning, we have found that the add-on DO storage volumes are half the speed of the base disk:
root@vol:/# dd bs=1M count=512 if=/dev/zero of=test conv=fdatasync
536870912 bytes (537 MB) copied, 1.72333 s, 312 MB/s
root@server:/var/discourse# dd bs=1M count=512 if=/dev/zero of=test conv=fdatasync
536870912 bytes (537 MB) copied, 3.03458 s, 177 MB/s
For reference a here are numbers from a modern consumer M.2 PCI SSD drive:
536870912 bytes (537 MB) copied, 0.564705 s, 951 MB/s
And a plain old consumer SATA SSD, but a good one, like the Samsung 950 pro.. though to be fair most name brand-ish SATA SSDs are close to this as it is a mature market:
536870912 bytes (537 MB) copied, 1.20895 s, 444 MB/s
I looked up old results, and ye olde spinning rust hard drives deliver anywhere from 50 MB/sec to 110 MB/sec.