lease-read: discuss election timeout / lease window enlargement (deferred from #549)

[bootjp]

Background

Current Raft tick configuration:

• defaultTickInterval = 10 ms
• defaultElectionTick = 100 → election timeout 1 s
• Lease window = 1 s − leaseSafetyMargin (300 ms) = 700 ms (margin 30%)

TiKV reference: election timeout 10 s, max lease 9 s (margin 10%). The larger window absorbs GC pauses and apply-queue depth without consuming the safety margin.

Trade-off

• Larger election timeout → slower failover detection.
• PR Adjust raft tick intervals for stability #529 already shifted heartbeatTick 1→10 and electionTick 10→100 for stability.
• Another 10× shift would push failover detection into the 10 s range; may be unacceptable for the Redis-compatible latency profile.

Proposed action

• Decide an SLO for failover detection latency.
• If 1 s remains the SLO: keep current config and accept the 30% margin.
• If a 3-5 s SLO is acceptable: raise electionTick to 300-500 (3-5 s window), giving the 10% margin TiKV uses.

References: PR #549, PR #529.

[bootjp]

lease 修正と独立した運用 SLO の設計判断であり、issue にする筋合いではない。必要時に別途議論する。