No. You journal the changes to disk. You therefore only need disks fast enough to keep up with the journal. If you get into a situation where your disks can'

No. You journal the changes to disk. You therefore only need disks fast enough to keep up with the journal. If you get into a situation where your disks can't keep up with the journal, your site is probably big and popular enough that you can afford to hire DBAs to go from there.