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Re: [EXTERNAL] Re: WIP: WAL prefetch (another approach)

Stephen Frost <sfrost@snowman.net>

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From: Stephen Frost <sfrost@snowman.net>
To: Sait Talha Nisanci <Sait.Nisanci@microsoft.com>
Cc: Thomas Munro <thomas.munro@gmail.com>, Tomas Vondra <tomas.vondra@2ndquadrant.com>, Dmitry Dolgov <9erthalion6@gmail.com>, David Steele <david@pgmasters.net>, Andres Freund <andres@anarazel.de>, Alvaro Herrera <alvherre@2ndquadrant.com>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2020-08-27T18:26:42Z
Lists: pgsql-hackers 
Greetings,

* Sait Talha Nisanci (Sait.Nisanci@microsoft.com) wrote:
> OS version is Ubuntu 18.04.5 LTS.
> Filesystem is ext4 and block size is 4KB.

[...]

* Sait Talha Nisanci (Sait.Nisanci@microsoft.com) wrote:
> I have run some benchmarks for this patch. Overall it seems that there is a good improvement with the patch on recovery times:
> 
> The VMs I used have 32GB RAM, pgbench is initialized with a scale factor 3000(so it doesn’t fit to memory, ~45GB).
> 
> In order to avoid checkpoints during benchmark, max_wal_size(200GB) and checkpoint_timeout(200 mins) are set to a high value. 
> 
> The run is cancelled when there is a reasonable amount of WAL ( > 25GB). The recovery times are measured from the REDO logs.
> 
> I have tried combination of SSD, HDD, full_page_writes = on/off and max_io_concurrency = 10/50, the recovery times are as follows (in seconds):
> 
> 			       No prefetch	    |     Default prefetch values  |	      Default + max_io_concurrency = 50
> SSD, full_page_writes = on	852		301				197
> SSD, full_page_writes = off	1642		1359				1391
> HDD, full_page_writes = on	6027		6345				6390
> HDD, full_page_writes = off	738		275				192
> 
> Default prefetch values:
> -	Max_recovery_prefetch_distance = 256KB
> -	Max_io_concurrency = 10
> 
> It probably makes sense to compare each row separately as the size of WAL can be different.

Is WAL FPW compression enabled..?  I'm trying to figure out how, given
what's been shared here, that replaying 25GB of WAL is being helped out
by 2.5x thanks to prefetch in the SSD case.  That prefetch is hurting in
the HDD case entirely makes sense to me- we're spending time reading
pages from the HDD, which is entirely pointless work given that we're
just going to write over those pages entirely with FPWs.

Further, if there's 32GB of RAM, and WAL compression isn't enabled and
the WAL is only 25GB, then it's very likely that every page touched by
the WAL ends up in memory (shared buffers or fs cache), and with FPWs we
shouldn't ever need to actually read from the storage to get those
pages, right?  So how is prefetch helping so much..?

I'm not sure that the 'full_page_writes = off' tests are very
interesting in this case, since you're going to get torn pages and
therefore corruption and hopefully no one is running with that
configuration with this OS/filesystem.

Thanks,

Stephen

Commits

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

  1. Fix recovery_prefetch docs.

  2. Prefetch data referenced by the WAL, take II.

  3. Add circular WAL decoding buffer, take II.

  4. Fix generation of ./INSTALL for the distribution tarball

  5. Revert recovery prefetching feature.

  6. Sync guc.c and postgresql.conf.sample with the SGML docs.

  7. Add information of total data processed to replication slot stats.

  8. Doc: Review for "Optionally prefetch referenced data in recovery."

  9. Add circular WAL decoding buffer.

  10. Optionally prefetch referenced data in recovery.

  11. Remove read_page callback from XLogReader.

  12. Provide ReadRecentBuffer() to re-pin buffers by ID.

  13. Provide recovery_init_sync_method=syncfs.

  14. Mark factorial operator, and postfix operators in general, as deprecated.

  15. Rationalize GetWalRcv{Write,Flush}RecPtr().

  16. Support PrefetchBuffer() in recovery.

  17. Prevent hard failures of standbys caused by recycled WAL segments