Horton

Re: refactoring basebackup.c

Jeevan Ladhe <jeevan.ladhe@enterprisedb.com>

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From: Jeevan Ladhe <jeevan.ladhe@enterprisedb.com>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Mark Dilger <mark.dilger@enterprisedb.com>, "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>, tushar <tushar.ahuja@enterprisedb.com>
Date: 2021-09-21T13:07:37Z
Lists: pgsql-hackers

Commits

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

  1. Document BaseBackupSync and BaseBackupWrite wait events.

  2. Support long distance matching for zstd compression

  3. Fix possible NULL-pointer-deference in backup_compression.c.

  4. Allow parallel zstd compression when taking a base backup.

  5. Make PostgreSQL::Test::Cluster::run_log() return a useful value.

  6. Fix a few goofs in new backup compression code.

  7. Replace BASE_BACKUP COMPRESSION_LEVEL option with COMPRESSION_DETAIL.

  8. Add 'basebackup_to_shell' contrib module.

  9. Allow extensions to add new backup targets.

  10. Change HAVE_LIBLZ4 and HAVE_LIBZSTD tests to USE_LZ4 and USE_ZSTD.

  11. pg_basebackup: Clean up some bogus file extension tests.

  12. pg_basebackup: Avoid unclean failure with server-compression and -D -.

  13. Fix LZ4 tests for remaining buffer space.

  14. Add support for zstd base backup compression.

  15. pg_basebackup: Allow client-side LZ4 (de)compression.

  16. Add suport for server-side LZ4 base backup compression.

  17. Add min() and max() aggregates for xid8.

  18. Remove superfluous variable.

  19. pg_basebackup: Cleaner handling when compression is multiply specified.

  20. Allow server-side compression to be used with -Fp.

  21. pg_basebackup: Fix a couple of recently-introduced bugs.

  22. Tidy up a few cosmetic issues related to pg_basebackup.

  23. Server-side gzip compression.

  24. Unbreak pg_basebackup/t/010_pg_basebackup.pl on msys

  25. Suppress variable-set-but-not-used warning from clang 13.

  26. Extend the options of pg_basebackup to control compression

  27. Support base backup targets.

  28. Modify pg_basebackup to use a new COPY subprotocol for base backups.

  29. Document that tar archives are now properly terminated.

  30. Fix thinko in bbsink_throttle_manifest_contents.

  31. Have the server properly terminate tar archives.

  32. Minimal fix for unterminated tar archive problem.

  33. Introduce 'bbstreamer' abstraction to modularize pg_basebackup.

  34. Introduce 'bbsink' abstraction to modularize base backup code.

  35. Refactor basebackup.c's _tarWriteDir() function.

  36. Flexible options for CREATE_REPLICATION_SLOT.

  37. Flexible options for BASE_BACKUP.

>
> >> + /*
> >> + * LZ4F_compressUpdate() returns the number of bytes written into
> output
> >> + * buffer. We need to keep track of how many bytes have been
> cumulatively
> >> + * written into the output buffer(bytes_written). But,
> >> + * LZ4F_compressUpdate() returns 0 in case the data is buffered and not
> >> + * written to output buffer, set autoFlush to 1 to force the writing
> to the
> >> + * output buffer.
> >> + */
> >> + prefs->autoFlush = 1;
> >>
> >> I don't see why this should be necessary. Elsewhere you have code that
> >> caters to bytes being stuck inside LZ4's buffer, so why do we also
> >> require this?
> >
> > This is needed to know the actual bytes written in the output buffer. If
> it is
> > set to 0, then LZ4F_compressUpdate() would randomly return 0 or actual
> > bytes are written to the output buffer, depending on whether it has
> buffered
> > or really flushed data to the output buffer.
>
> The problem is that if we autoflush, I think it will cause the
> compression ratio to be less good. Try un-lz4ing a file that is
> produced this way and then re-lz4 it and compare the size of the
> re-lz4'd file to the original one. Compressors rely on postponing
> decisions about how to compress until they've seen as much of the
> input as possible, and flushing forces them to decide earlier, and
> maybe making a decision that isn't as good as it could have been. So I
> believe we should look for a way of avoiding this. Now I realize
> there's a problem there with doing that and also making sure the
> output buffer is large enough, and I'm not quite sure how we solve
> that problem, but there is probably a way to do it.
>

Yes, you are right here, and I could verify this fact with an experiment.
When autoflush is 1, the file gets less compressed i.e. the compressed file
is of more size than the one generated when autoflush is set to 0.
But, as of now, I couldn't think of a solution as we need to really advance
the
bytes written to the output buffer so that we can write into the output
buffer.

Regards,
Jeevan Ladhe