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
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