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Re: WIP: WAL prefetch (another approach)

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Tom Lane <tgl@sss.pgh.pa.us>, Kyotaro HORIGUCHI <horiguchi.kyotaro@lab.ntt.co.jp>
Cc: Thomas Munro <thomas.munro@gmail.com>, Stephen Frost <sfrost@snowman.net>, Alvaro Herrera <alvherre@2ndquadrant.com>, David Steele <david@pgmasters.net>, Dmitry Dolgov <9erthalion6@gmail.com>, Jakub Wartak <Jakub.Wartak@tomtom.com>, Tomas Vondra <tomas.vondra@enterprisedb.com>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2021-04-29T02:25:53Z
Lists: pgsql-hackers

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Hi,

On 2021-04-28 17:59:22 -0700, Andres Freund wrote:
> I can however say that pg_waldump on the standby's pg_wal does also
> fail. The failure as part of the backend is "invalid memory alloc
> request size", whereas in pg_waldump I get the much more helpful:
> pg_waldump: fatal: error in WAL record at 4/7F5C31C8: record with incorrect prev-link 416200FF/FF000000 at 4/7F5C3200

There's definitely something broken around continuation records, in
XLogFindNextRecord(). Which means that it's not the cause for the server
side issue, but obviously still not good.

The conversion of XLogFindNextRecord() to be state machine based
basically only works in a narrow set of circumstances. Whenever the end
of the first record read is on a different page than the start of the
record, we'll endlessly loop.

We'll go into XLogFindNextRecord(), and return until we've successfully
read the page header. Then we'll enter the second loop. Which will try
to read until the end of the first record. But after returning the first
loop will again ask for page header.

Even if that's fixed, the second loop alone has the same problem: As
XLogBeginRead() is called unconditionally we'll start read the start of
the record, discover that it needs data on a second page, return, and
do the same thing again.

I think it needs something roughly like the attached.

Greetings,

Andres Freund

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