21st Century Keykos Checkpoints

I have worried about SSD technology in regard to Keykos use of disks. Just now (2016 December) I thought of something that should be recorded. I would have updated our principle checkpoint document but that seems to be Charlie Landau’s authoritative description.

There are two sorts of SSD, one is cheaper but has limits on how many writes a storage block may endure. The other is more expensive but has a much larger limit, probably competing with disks. The new idea is to use the high write SSD for swapping and the cheap SSD for the home locations. Both are non-volatile. Writes to the swapping are are much more frequent than to the home areas.

Very crudely this suggests that you might hope to write 10¹⁵ bytes on the cheap SSD. Migration, which is the only thing that wrote home positions, was far from a full time activity of the 370 disk channels. One mega-byte per second would be a high estimate for supporting dozens of program developers. At this rate it would take 10⁹ seconds or 30 years to exhaust the SSD. Some work loads will be much heavier, however.