Re: Different compression methods for FPI

On Wed, Jun 16, 2021 at 11:49:51AM +0300, Heikki Linnakangas wrote:
> Hmm, do we currently compress each block in a WAL record separately, for
> records that contain multiple full-page images? That could make a big
> difference e.g. for GiST index build that WAL-logs 32 pages in each record.
> If it helps the compression, we should probably start WAL-logging b-tree
> index build in larger batches, too.

Each block is compressed alone, see XLogCompressBackupBlock() in
XLogRecordAssemble() where we loop through each block. Compressing a
group of blocks would not be difficult (the refactoring may be
trickier than it looks) but I am wondering how we should treat the
case where we finish by not compressing a group of blocks as there is
a safety fallback to not enforce a failure if a block cannot be
compressed. Should we move back to the compression of individual
blocks or just log all those pages uncompressed without their holes?
I really don't expect a group of blocks to not be compressed, just
being a bit paranoid here about the fallback we'd better have.
--
Michael