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

Robert Haas <robertmhaas@gmail.com>

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From: Robert Haas <robertmhaas@gmail.com>
To: Stephen Frost <sfrost@snowman.net>
Cc: Andres Freund <andres@anarazel.de>, Sait Talha Nisanci <Sait.Nisanci@microsoft.com>, Thomas Munro <thomas.munro@gmail.com>, Tomas Vondra <tomas.vondra@2ndquadrant.com>, Dmitry Dolgov <9erthalion6@gmail.com>, David Steele <david@pgmasters.net>, Alvaro Herrera <alvherre@2ndquadrant.com>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2020-08-27T20:19:23Z
Lists: pgsql-hackers 
On Thu, Aug 27, 2020 at 2:51 PM Stephen Frost <sfrost@snowman.net> wrote:
> > Hm? At least earlier versions didn't do prefetching for records with an fpw, and only for subsequent records affecting the same or if not in s_b anymore.
>
> We don't actually read the page when we're replaying an FPW though..?
> If we don't read it, and we entirely write the page from the FPW, how is
> pre-fetching helping..?

Suppose there is a checkpoint. Then we replay a record with an FPW,
pre-fetching nothing. Then the buffer gets evicted from
shared_buffers, and maybe the OS cache too. Then, before the next
checkpoint, we again replay a record for the same page. At this point,
pre-fetching should be helpful.

Admittedly, I don't quite understand whether that is what is happening
in this test case, or why SDD vs. HDD should make any difference. But
there doesn't seem to be any reason why it doesn't make sense in
theory.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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