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: 2022-04-08T04:44:43Z
Lists: pgsql-hackers 
At Thu, 7 Apr 2022 20:59:21 -0700, Andres Freund <andres@anarazel.de> wrote in 
> Hi,
> 
> On 2022-04-08 11:10:14 +0900, Kyotaro Horiguchi wrote:
> > 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>

Looks perfect generally, and especially in regard to the concern.

> I think I like my version above a bit better?

Quite a bit.  It didn't answer for the concern.

> > > 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>

Looks clearer since it mention user-facing interfaces with concrete
example columns.

regards.

-- 
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.