Horton

Re: shared-memory based stats collector - v70

Kyotaro Horiguchi <horikyota.ntt@gmail.com>

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From: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
To: andres@anarazel.de
Cc: melanieplageman@gmail.com, pryzby@telsasoft.com, thomas.munro@gmail.com, david.g.johnston@gmail.com, pgsql-hackers@lists.postgresql.org
Date: 2022-04-08T02:10:14Z
Lists: pgsql-hackers 
At Thu, 7 Apr 2022 16:37:51 -0700, Andres Freund <andres@anarazel.de> wrote in 
> Hi,
> 
> On 2022-04-07 00:28:45 -0700, Andres Freund wrote:
> > I've gotten through the main commits (and then a fix for the apparently
> > inevitable bug that's immediately highlighted by the buildfarm), and the first
> > test. I'll call it a night now, and work on the other tests & docs tomorrow.
> 
> I've gotten through the tests now. There's one known, not yet addressed, issue
> with the stats isolation test, see [1].
> 
> 
> Working on the docs. Found a few things worth raising:
> 
> 1)
> Existing text:
>    When the server shuts down cleanly, a permanent copy of the statistics
>    data is stored in the <filename>pg_stat</filename> subdirectory, so that
>    statistics can be retained across server restarts.  When recovery is
>    performed at server start (e.g., after immediate shutdown, server crash,
>    and point-in-time recovery), all statistics counters are reset.
> 
> The existing docs patch hadn't updated yet. My current edit is
> 
>    When the server shuts down cleanly, a permanent copy of the statistics
>    data is stored in the <filename>pg_stat</filename> subdirectory, so that
>    statistics can be retained across server restarts.  When crash recovery is
>    performed at server start (e.g., after immediate shutdown, server crash,
>    and point-in-time recovery, but not when starting a standby that was shut
>    down normally), all statistics counters are reset.
> 
> but I'm not sure the parenthetical is easy enough to understand?

I can read it. But I'm not sure that the difference is obvious for
average users between "starting a standby from a basebackup" and
"starting a standby after a normal shutdown"..

Other than that, it might be easier to read if the additional part
were moved out to the end of the paragraph, prefixing with "Note:
". For example,

...
statistics can be retained across server restarts.  When crash recovery is
performed at server start (e.g., after immediate shutdown, server crash,
and point-in-time recovery), all statistics counters are reset. Note that
crash recovery is not performed when starting a standby that was shut
down normally then all counters are retained.

> 2)
> The edit is not a problem, but it's hard to understand what the existing
> paragraph actually means?
> 
> diff --git a/doc/src/sgml/high-availability.sgml b/doc/src/sgml/high-availability.sgml
> index 3247e056663..8bfb584b752 100644
> --- a/doc/src/sgml/high-availability.sgml
> +++ b/doc/src/sgml/high-availability.sgml
> @@ -2222,17 +2222,17 @@ HINT:  You can then restart the server after making the necessary configuration
> ...
>     <para>
> -    The statistics collector is active during recovery. All scans, reads, blocks,
> +    The cumulative statistics system is active during recovery. All scans, reads, blocks,
>      index usage, etc., will be recorded normally on the standby. Replayed
>      actions will not duplicate their effects on primary, so replaying an
>      insert will not increment the Inserts column of pg_stat_user_tables.
>      The stats file is deleted at the start of recovery, so stats from primary
>      and standby will differ; this is considered a feature, not a bug.
>     </para>
> 
>     <para>

Agreed partially. It's too detailed.  It might not need to mention WAL
replay.

> I'll just commit the necessary bit, but we really ought to rephrase this.
> 
> 
> 
> 
> Greetings,
> 
> Andres Freund
> 
> [1] https://www.postgresql.org/message-id/20220407165709.jgdkrzqlkcwue6ko%40alap3.anarazel.de

-- 
Kyotaro Horiguchi
NTT Open Source Software Center

Commits

  1. Improve comment about dropped entries in pgstat.c

  2. Fix temporary memory leak in system table index scans

  3. pgstat: set timestamps of fixed-numbered stats after a crash.

  4. pgstat: Update docs to match the shared memory stats reality.

  5. pgstat: Hide instability in stats.spec with -DCATCACHE_FORCE_RELEASE.

  6. pgstat: add/extend tests for resetting various kinds of stats.

  7. Add minimal tests for recovery conflict handling.

  8. pgstat: test stats interactions with physical replication.

  9. pgstat: add tests for handling of restarts, including crashes.

  10. pgstat: add tests for transaction behaviour, 2PC, function stats.

  11. pgstat: add pg_stat_have_stats() test helper.

  12. pgstat: add pg_stat_force_next_flush(), use it to simplify tests.

  13. pgstat: move pgstat.c to utils/activity.

  14. pgstat: store statistics in shared memory.

  15. pgstat: remove stats_temp_directory.

  16. pgstat: rename STATS_COLLECTOR GUC group to STATS_CUMULATIVE.

  17. pgstat: revise replication slot API in preparation for shared memory stats.

  18. pgstat: scaffolding for transactional stats creation / drop.

  19. pgstat: introduce PgStat_Kind enum.

  20. pgstat: prepare APIs used by pgstatfuncs for shared memory stats.

  21. pgstat: add pgstat_copy_relation_stats().

  22. pgstat: rename some pgstat_send_* functions to pgstat_report_*.

  23. pgstat: stats collector references in comments.

  24. pgstat: move transactional code into pgstat_xact.c.

  25. dsm: allow use in single user mode.

  26. dshash: revise sequential scan support.

  27. pgstat: remove some superflous comments from pgstat.h.

  28. pgstat: reorder pgstat.[ch] contents.

  29. pgstat: split different types of stats into separate files.

  30. pgstat: introduce pgstat_relation_should_count().

  31. pgstat: separate "xact level" handling out of relation specific functions.

  32. pgstat: rename pgstat_initstats() to pgstat_relation_init().

  33. pgstat: split out WAL handling from pgstat_{initialize,report_stat}.

  34. pgstat: run pgindent on pgstat.c/h.

  35. pgstat: split relation, database handling out of pgstat_report_stat().

  36. Move code around in StartupXLOG().

  37. pgstat: Prepare to use mechanism for truncated rels also for droppped rels.

  38. pgstat: Split out relation stats handling from AtEO[Sub]Xact_PgStat() etc.

  39. pgstat: Schedule per-backend pgstat shutdown via before_shmem_exit().

  40. Schedule ShutdownXLOG() in single user mode using before_shmem_exit().

  41. Make parallel worker shutdown complete entirely via before_shmem_exit().

  42. pgstat: Bring up pgstat in BaseInit() to fix uninitialized use of pgstat by AV.

  43. pgstat: split reporting/fetching of bgwriter and checkpointer stats.

  44. Split backend status and progress related functionality out of pgstat.c.

  45. Split wait event related code from pgstat.[ch] into wait_event.[ch].

  46. Make archiver process an auxiliary process.

  47. Force to send remaining WAL stats to the stats collector at walwriter exit.

  48. Add pg_stat_database counters for sessions and session time

  49. Collect statistics about SLRU caches

  50. Don't run atexit callbacks in quickdie signal handlers.

  51. Create a "fast path" for acquiring weak relation locks.