Horton

Re: Add LZ4 compression in pg_dump

Justin Pryzby <pryzby@telsasoft.com>

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From: Justin Pryzby <pryzby@telsasoft.com>
To: gkokolatos@pm.me
Cc: Michael Paquier <michael@paquier.xyz>, pgsql-hackers@lists.postgresql.org, Rachel Heaton <rachelmheaton@gmail.com>
Date: 2023-01-08T19:45:25Z
Lists: pgsql-hackers 
On Thu, Dec 22, 2022 at 11:08:59AM -0600, Justin Pryzby wrote:
> There's a couple of lz4 bits which shouldn't be present in 002: file
> extension and comments.

There were "LZ4" comments and file extension stuff in the preparatory
commit.  But now it seems like you *removed* them in the LZ4 commit
(where it actually belongs) rather than *moving* it from the
prior/parent commit *to* the lz4 commit.  I recommend to run something
like "git diff @{1}" whenever doing this kind of patch surgery.

+   if (AH->compression_spec.algorithm != PG_COMPRESSION_NONE &&
+       AH->compression_spec.algorithm == PG_COMPRESSION_GZIP &&

This looks wrong/redundant.  The gzip part should be removed, right ?

Maybe other places that check if (compression==PG_COMPRESSION_GZIP)
should maybe change to say compression!=NONE?

_PrepParallelRestore() references ".gz", so I think it needs to be
retrofitted to handle .lz4.  Ideally, that's built into a struct or list
of file extensions to try.  Maybe compression.h should have a function
to return the file extension of a given algorithm.  I'm planning to send
a patch for zstd, and hoping its changes will be minimized by these
preparatory commits.

+ errno = errno ? : ENOSPC;

"?:" is a GNU extension (not the ternary operator, but the ternary
operator with only 2 args).  It's not in use anywhere else in postgres.
You could instead write it with 3 "errno"s or as "if (errno==0):
errno=ENOSPC"

You wrote "eol_flag == false" and "eol_flag == 0" and true.  But it's
cleaner to test it as a boolean: if (eol_flag) / if (!eol_flag).

Both LZ4File_init() and its callers check "inited".  Better to do it in
one place than 3.  It's a static function, so I think there's no
performance concern.

Gzip_close() still has a useless save_errno (or rebase issue?).

I think it's confusing to have two functions, one named
InitCompressLZ4() and InitCompressorLZ4().

pg_compress_specification is being passed by value, but I think it
should be passed as a pointer, as is done everywhere else.

pg_compress_algorithm is being writen directly into the pg_dump header.
Currently, I think that's not an externally-visible value (it could be
renumbered, theoretically even in a minor release).  Maybe there should
be a "private" enum for encoding the pg_dump header, similar to
WAL_COMPRESSION_LZ4 vs BKPIMAGE_COMPRESS_LZ4 ?  Or else a comment there
should warn that the values are encoded in pg_dump, and must never be
changed.

+ Verify that data files where compressed
typo: s/where/were/

Also:
s/occurance/occurrence/
s/begining/beginning/
s/Verfiy/Verify/
s/nessary/necessary/

BTW I noticed that cfdopen() was accidentally committed to compress_io.h
in master without being defined anywhere.

-- 
Justin

Commits

  1. Advance input pointer when LZ4 compressing data

  2. Null-terminate the output buffer of LZ4Stream_gets

  3. Rework code defining default compression for dir/custom formats in pg_dump

  4. pg_dump: Use only LZ4 frame format for compression

  5. Minor comment improvements for compress_lz4

  6. Unify buffer sizes in pg_dump compression API

  7. Improve type handling in pg_dump's compress file API

  8. Improve wording in pg_dump compression docs

  9. Fix condition in pg_dump TAP test

  10. Add LZ4 compression to pg_dump

  11. Introduce a generic pg_dump compression API

  12. Prepare pg_dump internals for additional compression methods

  13. Fix behavior with pg_restore -l and compressed dumps

  14. Add 250c8ee07ed to git-blame-ignore-revs

  15. Provide test coverage in pg_dump for default behaviors with compression

  16. Switch pg_dump to use compression specifications

  17. Refactor code parsing compression option values (-Z/--compress)

  18. meson: Add some missing env settings for tests of pg_dump and pg_verifybackup

  19. Extend TAP tests of pg_dump to test for compression with gzip

  20. Clean up some dead code in pg_dump with tar format and gzip compression

  21. Add TAP test in pg_dump with --format=tar and --compress

  22. Replace BASE_BACKUP COMPRESSION_LEVEL option with COMPRESSION_DETAIL.

  23. Refactor the pg_dump zlib code from pg_backup_custom.c to a separate file,