Horton

Re: refactoring basebackup.c

Robert Haas <robertmhaas@gmail.com>

View thread
From: Robert Haas <robertmhaas@gmail.com>
To: Jeevan Ladhe <jeevan.ladhe@enterprisedb.com>
Cc: Mark Dilger <mark.dilger@enterprisedb.com>, "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>, tushar <tushar.ahuja@enterprisedb.com>
Date: 2021-09-08T19:39:42Z
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.

On Wed, Sep 8, 2021 at 2:14 PM Jeevan Ladhe
<jeevan.ladhe@enterprisedb.com> wrote:
> To give an example, I put some logging statements, and I can see in the log:
> "
> bytes remaining in mysink->base.bbs_next->bbs_buffer: 16537
> input size to be compressed: 512
> estimated size for compressed buffer by LZ4F_compressBound(): 262667
> actual compressed size: 16
> "

That is pretty lame. I don't know why it needs a ~256k buffer to
produce 16 bytes of output.

The way the gzip APIs I used work, you tell it how big the output
buffer is and it writes until it fills that buffer, or until the input
buffer is empty, whichever happens first. But this seems to be the
other way around: you tell it how much input you have, and it tells
you how big a buffer it needs. To handle that elegantly, I think I
need to make some changes to the design of the bbsink stuff. What I'm
thinking is that each bbsink somehow tells the next bbsink how big to
make the buffer. So if the LZ4 buffer is told that its buffer should
be at least, I don't know, say 64kB. Then it can compute how large an
output buffer the LZ4 library requires for 64kB. Hopefully we can
assume that liblz4 never needs a smaller buffer for a larger input.
Then we can assume that if a 64kB input requires, say, a 300kB output
buffer, every possible input < 64kB also requires an output buffer <=
300 kB.

But we can't just say, well, we were asked to create a 64kB buffer (or
whatever) so let's ask the next bbsink for a 300kB buffer (or
whatever), because then as soon as we write any data at all into it
the remaining buffer space might be insufficient for the next chunk.
So instead what I think we should do is have bbsink_lz4 set the size
of the next sink's buffer to its own buffer size +
LZ4F_compressBound(its own buffer size). So in this example if it's
asked to create a 64kB buffer and LZ4F_compressBound(64kB) = 300kB
then it asks the next sink to set the buffer size to 364kB. Now, that
means that there will always be at least 300 kB available in the
output buffer until we've accumulated a minimum of 64 kB of compressed
data, and then at that point we can flush.

I think this would be relatively clean and would avoid the need for
the double copying that the current design forced you to do. What do
you think?

+ /*
+ * If we do not have enough space left in the output buffer for this
+ * chunk to be written, first archive the already written contents.
+ */
+ if (nextChunkLen > mysink->base.bbs_next->bbs_buffer_length -
mysink->bytes_written ||
+ mysink->bytes_written >= mysink->base.bbs_next->bbs_buffer_length)
+ {
+ bbsink_archive_contents(sink->bbs_next, mysink->bytes_written);
+ mysink->bytes_written = 0;
+ }

I think this is flat-out wrong. It assumes that the compressor will
never generate more than N bytes of output given N bytes of input,
which is not true. Not sure there's much point in fixing it now
because with the changes described above this code will have to change
anyway, but I think it's just lucky that this has worked for you in
your testing.

+ /*
+ * 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?

Thanks for researching this!

-- 
Robert Haas
EDB: http://www.enterprisedb.com