> Consider a scenario like this, > > Server A: primary > Server B :replica of A > Server C :replica of B > > and somehow A down ,so B gets promoted. > Server A: down > Server B :new primary > Server C :replica of B > > In this case, pg_rewind can be used to reconstruct the cascade; the source is C and the target is A. > However, we get error as belows by running pg_rewind. > > ``` > pg_rewind: fetched file "global/pg_control", length 8192 > pg_rewind: source and target cluster are on the same timeline > pg_rewind: no rewind required > ```
To fix the above mentioned behavior of pg_rewind, I suggest to change the
cascade standby's (i.e. server C's) minRecoveryPointTLI when it receives
the new timeline information from the new primary (i.e. server B).
When server B is promoted, it creates an end-of-recovery record by calling
CreateEndOfRecoveryRecord(). (in xlog.c)
And also updates B's minRecoveryPoint and minRecoveryPointTLI.
```
/*
* Update the control file so that crash recovery can follow the
timeline
* changes to this point.
*/
LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
ControlFile->minRecoveryPoint = recptr;
ControlFile->minRecoveryPointTLI = xlrec.ThisTimeLineID;
UpdateControlFile();
LWLockRelease(ControlFileLock);
```
Since C is a replica of B, the end-of-recovery record is replicated from B
to C, so the record is replayed in C by xlog_redo().
The attached patch updates minRecoveryPoint and minRecoveryPointTLI at this
point by mimicking CreateEndOfRecoveryRecord().
With this patch, you can run pg_rewind with cascade standby immediately.
(without waiting for checkpointing)
Thoughts?
Masaki Kuwamura
v1-0001-pg_rewind-Fix-bug-using-cascade-standby-as-source.patch
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