haumea/zrepl: reduce snapshot count

[vcunat]

Reduce snapshot count. We repeatedly run out of space on Haumea.

[vcunat]

Let me dump a bit about why.

The problematic situations that we see have lots of data unique to (some) in-between snapshots, i.e. dropping some of those snapshots (manually) could release lots of space. Consequently:

• making/transferring snapshots less often should decrease the total transfer amount and reduce this pressure on the remote
• larger spacing between snapshots kept on Haumea should decrease the total space needed there, perhaps even if we didn't significantly decrease the total time span covered by snapshots

EDIT:

• interesting note is that those problematic snapshots seem to have also larger total size (i.e. if we didn't have any snapshotting, the disk usage would be larger at those points)

[vcunat]

🤔 as for the backup location(s), it feels wasteful to keep every week uniformly for a year. Can you see any reason for it? I'd intuitively again go for some exponentially increasing spacing. I assume we can afford more space than on Haumea itself, so e.g. this slower one?

              "2x1h"
              "2x2h"
              "2x4h"
              "4x8h"
              # At this point the grid spans 2 days (-2h) by 10 snapshots.
              # (See note above about 8h -> 24h.)
              "2x1d"
              "2x2d"
              "2x4d"
              "2x8d"
              "2x16d"
              "2x32d"
              "2x64d"
              "2x128d"
              # At this point we keep 26 snapshots spanning 384--512 days (depends on moment),
              # with exponentially increasing spacing (almost).

Perhaps note the docs that the specified intervals do not overlap. All the intervals are stacked in the specified order and multiplicity, forming a fixed grid.