Horton

Re: Add LZ4 compression in pg_dump

Tomas Vondra <tomas.vondra@enterprisedb.com>

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From: Tomas Vondra <tomas.vondra@enterprisedb.com>
To: Justin Pryzby <pryzby@telsasoft.com>, gkokolatos@pm.me
Cc: Michael Paquier <michael@paquier.xyz>, shiy.fnst@fujitsu.com, pgsql-hackers@lists.postgresql.org, Rachel Heaton <rachelmheaton@gmail.com>
Date: 2023-03-01T16:39:54Z
Lists: pgsql-hackers 

On 2/27/23 05:49, Justin Pryzby wrote:
> 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
> 

Thanks. Those are definitely interesting improvements/optimizations!

I suggest we track them as a separate patch series - please add them to
the CF app (I guess you'll have to add them to 2023-07 at this point,
but we can get them in, I think).


regards

-- 
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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,