Horton

Re: Vacuum ERRORs out considering freezing dead tuples from before OldestXmin

Peter Geoghegan <pg@bowt.ie>

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From: Peter Geoghegan <pg@bowt.ie>
To: Melanie Plageman <melanieplageman@gmail.com>
Cc: Pg Hackers <pgsql-hackers@postgresql.org>, Andres Freund <andres@anarazel.de>, Noah Misch <noah@leadboat.com>
Date: 2024-06-21T00:02:16Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Test that vacuum removes tuples older than OldestXmin

  2. Lower minimum maintenance_work_mem to 64kB

  3. Add accidentally omitted test to meson build file

  4. Use DELETE instead of UPDATE to speed up vacuum test

  5. Revert "Test that vacuum removes tuples older than OldestXmin"

  6. Ensure vacuum removes all visibly dead tuples older than OldestXmin

On Thu, Jun 20, 2024 at 7:42 PM Melanie Plageman
<melanieplageman@gmail.com> wrote:
> If vacuum fails to remove a tuple with xmax older than
> VacuumCutoffs->OldestXmin and younger than
> GlobalVisState->maybe_needed, it will ERROR out when determining
> whether or not to freeze the tuple with "cannot freeze committed
> xmax".
>
> In back branches starting with 14, failing to remove tuples older than
> OldestXmin during pruning caused vacuum to infinitely loop in
> lazy_scan_prune(), as investigated on this [1] thread.

This is a great summary.

> We can fix this by always removing tuples considered dead before
> VacuumCutoffs->OldestXmin. This is okay even if a reconnected standby
> has a transaction that sees that tuple as alive, because it will
> simply wait to replay the removal until it would be correct to do so
> or recovery conflict handling will cancel the transaction that sees
> the tuple as alive and allow replay to continue.

I think that this is the right general approach.

> The repro forces a round of index vacuuming after the standby
> reconnects and before pruning a dead tuple whose xmax is older than
> OldestXmin.
>
> At the end of the round of index vacuuming, _bt_pendingfsm_finalize()
> calls GetOldestNonRemovableTransactionId(), thereby updating the
> backend's GlobalVisState and moving maybe_needed backwards.

Right. I saw details exactly consistent with this when I used GDB
against a production instance.

I'm glad that you were able to come up with a repro that involves
exactly the same basic elements, including index page deletion.

-- 
Peter Geoghegan