During review of: https://github.com/etcd-io/etcd/pull/12988 spotted
that PUT is actially writing to v3-backend.
If we are replaying WAL log, it might happened that backend's
applied_index is > than the WAL's log entry. In such situation we should
skip applying on backend V3.
I think both the methods (setVersion, setMembersAttributes) are in
practice idempotent so its not that 'serious' problem, but for
formal correctness adding the proper checks.
This makes (bbolt) backend a full feature snapshot in term of WAL/raft,
i.e. carries:
- commit : (applied_index)
- confState
Benefits:
- Backend will be a sufficient point in time definition sufficient to
start replaying WAL. We have applied_index & confState in consistent
state.
- In case of emergency a backend state can be used for recovery
Every transaction committed to backend is writing most recent consistent_index.
Makes sure that even automatically trigger commits of batch-transactions
stays "really" consistent a.d. the most recent WAL log index applied.
Prior to this CL, `ETCDCTL_API=2 etcdctl backup --with-v3` was readacting WAL log
(by removal of some entries), but was NOT updating consistent_index in the backend.
Also the WAL editing logic was buggy, as it didn't took in consideration the fact
that when TERM changes, there can be entries with duplicated indexes in
the log. So its NOT sufficient to subtract number of removed entries to
get accurate log indexes.
The PR replaces removing and shifting of WAL entries with replacing them with an no-op entries.
Thanks to this consistent-index references are staying up to date.
The PR also:
- updates 'verification' logic to check whether consistent_index does not lag befor last snapshot
- env-gated execution of verification framework in `etcdctl backup`.
Tested with:
```
(./build.sh && cd tests && EXPECT_DEBUG=TRUE 'env' 'go' 'test' '-timeout=300m' 'go.etcd.io/etcd/tests/v3/e2e' -run=TestCtlV2Backup --count=1000 2>&1 | tee TestCtlV2BackupV3.log)
```
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).
