## Problem
Right now if there are too many running xacts to be restored from CLOG
at replica startup,
then replica is not trying to restore them and wait for non-overflown
running-xacs WAL record from primary.
But if primary is not active, then replica will not start at all.
Too many running xacts can be caused by transactions with large number
of subtractions.
But right now it can be also cause by two reasons:
- Lack of shutdown checkpoint which updates `oldestRunningXid` (because
of immediate shutdown)
- nextXid alignment on 1024 boundary (which cause loosing ~1k XIDs on
each restart)
Both problems are somehow addressed now.
But we have existed customers with "sparse" CLOG and lack of
checkpoints.
To be able to start RO replicas for such customers I suggest to add GUC
which allows replica to start even in case of subxacts overflow.
## Summary of changes
Add `neon.running_xacts_overflow_policy` with the following values:
- ignore: restore from CLOG last N XIDs and accept connections
- skip: do not restore any XIDs from CXLOGbut still accept connections
- wait: wait non-overflown running xacts record from primary node
## Checklist before requesting a review
- [ ] I have performed a self-review of my code.
- [ ] If it is a core feature, I have added thorough tests.
- [ ] Do we need to implement analytics? if so did you add the relevant
metrics to the dashboard?
- [ ] If this PR requires public announcement, mark it with
/release-notes label and add several sentences in this section.
## Checklist before merging
- [ ] Do not forget to reformat commit message to not include the above
checklist
---------
Co-authored-by: Konstantin Knizhnik <knizhnik@neon.tech>
We have one pretty serious MVCC visibility bug with hot standby
replicas. We incorrectly treat any transactions that are in progress
in the primary, when the standby is started, as aborted. That can
break MVCC for queries running concurrently in the standby. It can
also lead to hint bits being set incorrectly, and that damage can last
until the replica is restarted.
The fundamental bug was that we treated any replica start as starting
from a shut down server. The fix for that is straightforward: we need
to set 'wasShutdown = false' in InitWalRecovery() (see changes in the
postgres repo).
However, that introduces a new problem: with wasShutdown = false, the
standby will not open up for queries until it receives a running-xacts
WAL record from the primary. That's correct, and that's how Postgres
hot standby always works. But it's a problem for Neon, because:
* It changes the historical behavior for existing users. Currently,
the standby immediately opens up for queries, so if they now need to
wait, we can breka existing use cases that were working fine
(assuming you don't hit the MVCC issues).
* The problem is much worse for Neon than it is for standalone
PostgreSQL, because in Neon, we can start a replica from an
arbitrary LSN. In standalone PostgreSQL, the replica always starts
WAL replay from a checkpoint record, and the primary arranges things
so that there is always a running-xacts record soon after each
checkpoint record. You can still hit this issue with PostgreSQL if
you have a transaction with lots of subtransactions running in the
primary, but it's pretty rare in practice.
To mitigate that, we introduce another way to collect the
running-xacts information at startup, without waiting for the
running-xacts WAL record: We can the CLOG for XIDs that haven't been
marked as committed or aborted. It has limitations with
subtransactions too, but should mitigate the problem for most users.
See https://github.com/neondatabase/neon/issues/7236.
Co-authored-by: Konstantin Knizhnik <knizhnik@neon.tech>
