Fangmin Lv created ZOOKEEPER-3657:
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Summary: Implementing snapshot schedule to avoid high latency
issue due to disk contention
Key: ZOOKEEPER-3657
URL: https://issues.apache.org/jira/browse/ZOOKEEPER-3657
Project: ZooKeeper
Issue Type: New Feature
Components: server
Reporter: Fangmin Lv
Assignee: Fangmin Lv
Fix For: 3.6.0
If ZK server is running a machine with single disk driver, the snapshot and txn
fsync thread will have disk IO contention (even on SSD). Majority taking
snapshot will affect the txn fsync time, and hence the end to end update and
read latency.
To provide better SLA guarantee and improve the write throughput with large
snapshot (> 3GB), the snapshot scheduler is implemented internally to avoid
majority taking snapshot at the same time, which provides better latency
guarantee.
A new quorum packet type SNAPPING is introduced in this feature, leader will
send this packet to the followers periodically like PING but less frequently.
Followers will send the current status back, like the maximum txns since last
snapshot, fsync latency, etc, and leader will decide who should take snapshot.
On follower, it will enable safe snapshot mode if leader is sending SNAPPING,
which will only take snapshot if the txns is much larger than the threshold we
defined for SyncRequestProcessor, this is used to avoid issues like the
follower accumulated too many txns before it is scheduled to take
snapshot, or there are 2/5 servers down for a long time, and the leader is not
issuing snap for a long time.
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