On Tue, Jun 21, 2022 at 4:22 PM Thomas Munro <thomas.mu...@gmail.com> wrote: > > On Tue, Jun 21, 2022 at 10:33 PM Jakub Wartak <jakub.war...@tomtom.com> wrote: > > > > Maybe the important question is why would be readahead mechanism be > > > disabled in the first place via /sys | blockdev ? > > > > > > Because database should know better than OS which data needs to be > > > prefetched and which should not. Big OS readahead affects index scan > > > performance. > > > > OK fair point, however the patch here is adding 1 syscall per XLOG_BLCKSZ > > which is not cheap either. The code is already hot and there is example > > from the past where syscalls were limiting the performance [1]. Maybe it > > could be prefetching in larger batches (128kB? 1MB? 16MB?) ? > > I've always thought we'd want to tell it about the *next* segment > file, to smooth the transition from one file to the next, something > like the attached (not tested).
Yes, it makes sense to prefetch the "future" WAL files that "may be" needed for recovery (crash recovery/archive or PITR recovery/standby recovery), not the current WAL file. Having said that, it's not a great idea (IMO) to make the WAL readers prefetching instead WAL prefetching can be delegated to a new background worker or existing bg writer or checkpointer which gets started during recovery. Also, it's a good idea to measure the benefits with and without WAL prefetching for all recovery types - crash recovery/archive or PITR recovery/standby recovery. For standby recovery, the WAL files may be in OS cache if there wasn't a huge apply lag. Regards, Bharath Rupireddy.
