storage: allocate a more conservative slice of raft log entries

[tamird]

Note that this currently doesn't reallocate if the raft log entry cache is empty.

Also, we should probably pass a pre-allocated slice into the raft log entry cache.

---

This change is 

[petermattis]

Review status: 0 of 1 files reviewed at latest revision, 2 unresolved discussions, some commit checks pending.

---

pkg/storage/replica_raftstorage.go, line 100 at r1 (raw file):

	// Reallocate `ents` if we expect to have many more log entries.
	if numEnts := uint64(len(ents)); numEnts > 0 {

I think there is a common case here where numEnts == 0.

---

pkg/storage/replica_raftstorage.go, line 101 at r1 (raw file):

	// Reallocate `ents` if we expect to have many more log entries.
	if numEnts := uint64(len(ents)); numEnts > 0 {
		expectedEnts := (hi - lo) * size / numEnts

This is computing expectedSize, right? To get expectedEnts I think you need to multiply by size / maxBytes. Or maybe I'm not thinking clearly here.

This feels overly complicated. I was thinking we'd do something like:

  n := hi-lo
  if n > 100 {
    n = 100
  }
  ents = append(make([]raftpb.Entry, 0, n), ents...)

---

Comments from Reviewable

[tamird]

Review status: 0 of 4 files reviewed at latest revision, 2 unresolved discussions.

---

pkg/storage/replica_raftstorage.go, line 100 at r1 (raw file):

Previously, petermattis (Peter Mattis) wrote…

I think there is a common case here where numEnts == 0.

Done.

---

pkg/storage/replica_raftstorage.go, line 101 at r1 (raw file):

Previously, petermattis (Peter Mattis) wrote…

This is computing expectedSize, right? To get expectedEnts I think you need to multiply by size / maxBytes. Or maybe I'm not thinking clearly here.

This feels overly complicated. I was thinking we'd do something like:

  n := hi-lo
  if n > 100 {
    n = 100
  }
  ents = append(make([]raftpb.Entry, 0, n), ents...)

Alright, done.

Also moved allocation outside of getEntries.

---

Comments from Reviewable

[petermattis]

---

Review status: 0 of 4 files reviewed at latest revision, all discussions resolved, some commit checks pending.

---

Comments from Reviewable