ClusterVersionSet, ClusterMemberAttrSet, DowngradeInfoSet functions are
writing both to V2store and backend. Prior this CL there were
in a branch not executed if shouldApplyV3 was false,
e.g. during restore when Backend is up-to-date (has high
consistency-index) while v2store requires replay from WAL log.
The most serious consequence of this bug was that v2store after restore
could have different index (revision) than the same exact store before restore,
so potentially different content between replicas.
Also this change is supressing double-applying of Membership
(ClusterConfig) changes on Backend (store v3) - that lackilly are not
part of MVCC/KeyValue store, so they didn't caused Revisions to be
bumped.
Inspired by jingyih@ comment:
https://github.com/etcd-io/etcd/pull/12820#issuecomment-815299406
Motivation:
- ServerConfig is part of 'embed' public API, while etcdserver is more 'internal'
- EtcdServer is already too big and config is pretty wide-spread leaf
if we were to split etcdserver (e.g. into pre & post-apply part).
The flag protects etcd memory from being swapped out to disk.
This can happen in memory constrained systems where mmaped bbolt
area is natural condidate for swapping out.
This flag should provide better tail latency on the cost of higher RSS
ram usage. If the experiment is successful, the logic should get moved
into bbolt layer, where we can protect specific bbolt instances
(e.g. avoid protecting both during defragmentation).
