Horton

Re: shared-memory based stats collector - v70

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Cc: melanieplageman@gmail.com, pryzby@telsasoft.com, thomas.munro@gmail.com, david.g.johnston@gmail.com, pgsql-hackers@lists.postgresql.org
Date: 2022-04-08T03:59:21Z
Lists: pgsql-hackers 
Hi,

On 2022-04-08 11:10:14 +0900, Kyotaro Horiguchi wrote:
> 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"..

Yea, that's what I was concerned about. How about:

  <para>
   Cumulative statistics are collected in shared memory. Every
   <productname>PostgreSQL</productname> process collects statistics locally
   then updates the shared data at appropriate intervals.  When a server,
   including a physical replica, 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.  In contrast,
   when starting from an unclean shutdown (e.g., after an immediate shutdown,
   a server crash, starting from a base backup, and point-in-time recovery),
   all statistics counters are reset.
  </para>


> 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.

I think I like my version above a bit better?


> > 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.

My concern is more that it seems halfway nonsensical. "Replayed actions will
not duplicate their effects on primary" - I can guess what that means, but not
more. There's no "Inserts" column of pg_stat_user_tables.


   <para>
    The cumulative statistics system is active during recovery. All scans,
    reads, blocks, index usage, etc., will be recorded normally on the
    standby. However, WAL replay will not increment relation and database
    specific counters. I.e. replay will not increment pg_stat_all_tables
    columns (like n_tup_ins), nor will reads or writes performed by the
    startup process be tracked in the pg_statio views, nor will associated
    pg_stat_database columns be incremented.
   </para>



Greetings,

Andres Freund

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.