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Re: pg_waldump: support decoding of WAL inside tarfile

Thomas Munro <thomas.munro@gmail.com>

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From: Thomas Munro <thomas.munro@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Tomas Vondra <tomas@vondra.me>, Andres Freund <andres@anarazel.de>, Michael Paquier <michael@paquier.xyz>, Andrew Dunstan <andrew@dunslane.net>, Amul Sul <sulamul@gmail.com>, Zsolt Parragi <zsolt.parragi@percona.com>, Robert Haas <robertmhaas@gmail.com>, Chao Li <li.evan.chao@gmail.com>, Anthonin Bonnefoy <anthonin.bonnefoy@datadoghq.com>, Fujii Masao <masao.fujii@gmail.com>, Jakub Wartak <jakub.wartak@enterprisedb.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2026-03-28T23:15:39Z
Lists: pgsql-hackers

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On Sun, Mar 29, 2026 at 11:37 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> I wrote:
> > However ... I do not find any indication in the GNU tar docs
> > that it produces sparse files by default.  It looks like you
> > need to say -S/--sparse to make that happen.  Maybe you have
> > a version that's been hacked to make that the default?
>
> Bleah.  Digging in the man pages at freebsd.org, I read
>
>     --read-sparse
>                (c, r, u modes only) Read sparse file  information  from  disk.
>                This  is the reverse of --no-read-sparse and the default behav-
>                ior.
>
> It's apparently been there and been default since FreeBSD 13.1.
> This leads one to wonder how come BF member dikkop is managing
> to run this test successfully.  I speculate that it's using a
> filesystem type that doesn't do sparse files (cc'ing Vondra
> for confirmation on that).
>
> It looks like to make this test stable on modern FreeBSD,
> we need to see if tar accepts --no-read-sparse and use that
> switch if so.

Yeah.  Here's my attempt at perl.

I think your Mac probably has a similar tar program BTW... but apfs
probably doesn't go around making holes visible to lseek()
automatically or at least as eagerly as my ZFS system.

Commits

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

  1. Use size_t instead of Size in pg_waldump

  2. More tar portability adjustments.

  3. Further harden tests that might use not-so-compatible tar versions.

  4. Harden astreamer tar parsing logic against archives it can't handle.

  5. Fix pg_waldump/t/001_basic.pl with BSD tar on ZFS.

  6. Remove a low-value, high-risk optimization in pg_waldump.

  7. Fix misuse of simplehash.h hash operations in pg_waldump.

  8. Fix file descriptor leakages in pg_waldump.

  9. Fix poorly-sized buffers in astreamer compression modules.

  10. Remove read_archive_file()'s "count" parameter.

  11. Report detailed errors from XLogFindNextRecord() failures.

  12. Fix assorted bugs in archive_waldump.c.

  13. Remove nonfunctional tar file trailer size check.

  14. Fix finalization of decompressor astreamers.

  15. Move tar detection and compression logic to common.

  16. pg_verifybackup: Enable WAL parsing for tar-format backups

  17. pg_waldump: Add support for reading WAL from tar archives

  18. pg_waldump: Preparatory refactoring for tar archive WAL decoding.

  19. pg_waldump: Remove file-level global WalSegSz.

  20. pg_verifybackup: Verify tar-format backups.