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Re: WIP: WAL prefetch (another approach)

Thomas Munro <thomas.munro@gmail.com>

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From: Thomas Munro <thomas.munro@gmail.com>
To: Stephen Frost <sfrost@snowman.net>
Cc: 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: 2020-07-02T03:09:29Z
Lists: pgsql-hackers 
On Sat, Jun 6, 2020 at 12:36 PM Stephen Frost <sfrost@snowman.net> wrote:
> * Tomas Vondra (tomas.vondra@2ndquadrant.com) wrote:
> > I wonder if we can collect some stats to measure how effective the
> > prefetching actually is. Ultimately we want something like cache hit
> > ratio, but we're only preloading into page cache, so we can't easily
> > measure that. Perhaps we could measure I/O timings in redo, though?
>
> That would certainly be interesting, particularly as this optimization
> seems likely to be useful on some platforms (eg, zfs, where the
> filesystem block size is larger than ours..) and less on others
> (traditional systems which have a smaller block size).

I know one way to get information about cache hit ratios without the
page cache fuzz factor: if you combine this patch with Andres's
still-in-development AIO prototype and tell it to use direct IO, you
get the undiluted truth about hits and misses by looking at the
"prefetch" and "skip_hit" columns of the stats view.  I'm hoping to
have a bit more to say about how this patch works as a client of that
new magic soon, but I also don't want to make this dependent on that
(it's mostly orthogonal, apart from the "how deep is the queue" part
which will improve with better information).

FYI I am still trying to reproduce and understand the problem Tomas
reported; more soon.

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