Horton

Re: New strategies for freezing, advancing relfrozenxid early

Jeff Davis <pgsql@j-davis.com>

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From: Jeff Davis <pgsql@j-davis.com>
To: Peter Geoghegan <pg@bowt.ie>
Cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2022-10-04T05:13:24Z
Lists: pgsql-hackers 
On Mon, 2022-10-03 at 20:11 -0700, Peter Geoghegan wrote:
> True. Though I think that a strong bias in the direction of advancing
> relfrozenxid by some amount (not necessarily by very many XIDs) still
> makes sense, especially when we're already freezing aggressively.

Take the case where you load a lot of data in one transaction. After
the loading transaction finishes, those new pages will soon be marked
all-visible.

In the future, vacuum runs will have to decide what to do. If a vacuum
decides to do an aggressive scan to freeze all of those pages, it may
be at some unfortunate time and disrupt the workload. But if it skips
them all, then it's just deferring the work until it runs up against
autovacuum_freeze_max_age, which might also be at an unfortunate time.

So how does your patch series handle this case? I assume there's some
mechanism to freeze a moderate number of pages without worrying about
advancing relfrozenxid?

Regards,
	Jeff Davis

Commits

  1. Revert "Add eager and lazy freezing strategies to VACUUM."

  2. Add eager and lazy freezing strategies to VACUUM.

  3. Refine the definition of page-level freezing.

  4. Avoid special XID snapshotConflictHorizon values.

  5. Add page-level freezing to VACUUM.

  6. Remove overzealous MultiXact freeze assertion.

  7. Refactor how VACUUM passes around its XID cutoffs.

  8. Deduplicate freeze plans in freeze WAL records.

  9. Set relfrozenxid to oldest extant XID seen by VACUUM.

  10. Only skip pages marked as clean in the visibility map, if the last 32

  11. Add vacuum_freeze_table_age GUC option, to control when VACUUM should