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>, Tomas Vondra <tomas.vondra@enterprisedb.com>, shiy.fnst@fujitsu.com, pgsql-hackers@lists.postgresql.org, Rachel Heaton <rachelmheaton@gmail.com>
Date: 2023-02-27T04:49:10Z
Lists: pgsql-hackers

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On Sat, Feb 25, 2023 at 08:05:53AM -0600, Justin Pryzby wrote:
> On Fri, Feb 24, 2023 at 11:02:14PM -0600, Justin Pryzby wrote:
> > 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.
> 
> One more - WriteDataToArchiveGzip() says:

One more again.

The LZ4 path is using non-streaming mode, which compresses each block
without persistent state, giving poor compression for -Fc compared with
-Fp.  If the data is highly compressible, the difference can be orders
of magnitude.

$ ./src/bin/pg_dump/pg_dump -h /tmp postgres -Z lz4 -Fp |wc -c
12351763
$ ./src/bin/pg_dump/pg_dump -h /tmp postgres -Z lz4 -Fc |wc -c
21890708

That's not true for gzip:

$ ./src/bin/pg_dump/pg_dump -h /tmp postgres -t t1 -Z gzip -Fc |wc -c
2118869
$ ./src/bin/pg_dump/pg_dump -h /tmp postgres -t t1 -Z gzip -Fp |wc -c
2115832

The function ought to at least use streaming mode, so each block/row
isn't compressioned in isolation.  003 is a simple patch to use
streaming mode, which improves the -Fc case:

$ ./src/bin/pg_dump/pg_dump -h /tmp postgres -Z lz4 -Fc |wc -c
15178283

However, that still flushes the compression buffer, writing a block
header, for every row.  With a single-column table, pg_dump -Fc -Z lz4
still outputs ~10% *more* data than with no compression at all.  And
that's for compressible data.

$ ./src/bin/pg_dump/pg_dump -h /tmp postgres -t t1 -Fc -Z lz4 |wc -c
12890296
$ ./src/bin/pg_dump/pg_dump -h /tmp postgres -t t1 -Fc -Z none |wc -c
11890296

I think this should use the LZ4F API with frames, which are buffered to
avoid outputting a header for every single row.  The LZ4F format isn't
compatible with the LZ4 format, so (unlike changing to the streaming
API) that's not something we can change in a bugfix release.  I consider
this an Opened Item.

With the LZ4F API in 004, -Fp and -Fc are essentially the same size
(like gzip).  (Oh, and the output is three times smaller, too.)

$ ./src/bin/pg_dump/pg_dump -h /tmp postgres -t t1 -Z lz4 -Fp |wc -c
4155448
$ ./src/bin/pg_dump/pg_dump -h /tmp postgres -t t1 -Z lz4 -Fc |wc -c
4156548

-- 
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,