Horton

Re: Add LZ4 compression in pg_dump

Justin Pryzby <pryzby@telsasoft.com>

View thread View on postgr.es
From: Justin Pryzby <pryzby@telsasoft.com>
To: gkokolatos@pm.me
Cc: Michael Paquier <michael@paquier.xyz>, Tomas Vondra <tomas.vondra@enterprisedb.com>, shiy.fnst@fujitsu.com, pgsql-hackers@lists.postgresql.org, Rachel Heaton <rachelmheaton@gmail.com>
Date: 2023-02-25T05:02:14Z
Lists: pgsql-hackers

Attachments

I have some fixes (attached) and questions while polishing the patch for
zstd compression.  The fixes are small and could be integrated with the
patch for zstd, but could be applied independently.

- I'm unclear about get_error_func().  That's called in three places
  from pg_backup_directory.c, after failures from write_func(), to
  supply an compression-specific error message to pg_fatal().  But it's
  not being used outside of directory format, nor for errors for other
  function pointers, or even for all errors in write_func().  Is there
  some reason why each compression method's write_func() shouldn't call
  pg_fatal() directly, with its compression-specific message ?

- I still think supports_compression() should be renamed, or made into a
  static function in the necessary file.  The main reason is that it's
  more clear what it indicates - whether compression is "implemented by
  pgdump" and not whether compression is "supported by this postgres
  build".  It also seems possible that we'd want to add a function
  called something like supports_compression(), indicating whether the
  algorithm is supported by the current build.  It'd be better if pgdump
  didn't subjugate that name.

- Finally, the "Nothing to do in the default case" comment comes from
  Michael's commit 5e73a6048:

+       /*
+        * Custom and directory formats are compressed by default with gzip when
+        * available, not the others.
+        */
+       if ((archiveFormat == archCustom || archiveFormat == archDirectory) &&
+               !user_compression_defined)
        {
 #ifdef HAVE_LIBZ
-               if (archiveFormat == archCustom || archiveFormat == archDirectory)
-                       compressLevel = Z_DEFAULT_COMPRESSION;
-               else
+               parse_compress_specification(PG_COMPRESSION_GZIP, NULL,
+                                                                        &compression_spec);
+#else
+               /* Nothing to do in the default case */
 #endif
-                       compressLevel = 0;
        }


As the comment says: for -Fc and -Fd, the compression is set to zlib, if
enabled, and when not otherwise specified by the user.

Before 5e73a6048, this set compressLevel=0 for -Fp and -Ft, *and* when
zlib was unavailable.

But I'm not sure why there's now an empty "#else".  I also don't know
what "the default case" refers to.

Maybe the best thing here is to move the preprocessor #if, since it's no
longer in the middle of a runtime conditional:

 #ifdef HAVE_LIBZ
+       if ((archiveFormat == archCustom || archiveFormat == archDirectory) &&
+               !user_compression_defined)
+               parse_compress_specification(PG_COMPRESSION_GZIP, NULL,
+                                            &compression_spec);
 #endif

...but that elicits a warning about "variable set but not used"...

-- 
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,