[9front] gefs: future work

[9front] gefs: future work

[ori]

While gefs is landed, it's far from finished.
There are a few chunks of future work that I would
like to get done:

- repair tools; right now, we can check for corruption,
  but more would be welcome. there's also nothing to fix
  corruption if it's detected.

- storing small files inline; there's no need to
  create a new block if the file is smaller than
  the maximum size of a value in the Bε tree.

- merging/annihilating messages early; right now,
  all messages flush all the way down to the leaf
  before being applied, but they could be merged
  early; eg, a delete could eliminate an insert,
  or a chain of mtime changes could be merged
  into one.

- modifying the root block buffer in-place;
  right now, the root block is copied for every
  upsertion. With some care, an upsert can modify
  the in-memory copy of the root block, which only
  needs to be serialized and flushed on sync or when
  it is full.

- custom snapshot timing; right now, snaps are either
  taken automatically on a hard-coded schedule, or not
  taken at all.

- microoptimization; right now, performance is ok,
  but could be faster; measure and optimize.

- doing more eager space reclaim; getting back space
  from deleted files is delayed, and we could do some
  work in the sweeper to reclaim the space early.

I plan to hack on these as time permits, but at a
fairly low priority. Patches are welcome.