Hi, On 2021-02-03 19:21:25 -0500, Bruce Momjian wrote: > On Wed, Feb 3, 2021 at 03:29:13PM -0800, Andres Freund wrote: > > Changing this is *completely* infeasible. In a lot of workloads it'd > > cause a *massive* explosion of WAL volume. Like quadratically. You'll > > need to find another way to generate a nonce. > > Do we often do multiple writes to the file system of the same page > during a single checkpoint, particularly only-hint-bit-modified pages? > I didn't think so.
It can easily happen. Consider ringbuffer using scans (like vacuum, seqscan) - they'll force the buffer out to disk soon after it's been dirtied. And often will read the same page again a short bit later. Or just any workload that's a bit bigger than shared buffers (but data is in the OS cache). Subsequent scans will often have new hint bits to set. > Is the logical approach here to modify XLogSaveBufferForHint() so if a > page write is not needed, to create a dummy WAL record that just > increments the WAL location and updates the page LSN? > (Is there a small WAL record I should reuse?) I think an explicit record type would be better. Or a hint record without an associated FPW. > I can try to add a hint-bit-page-write page counter, but that might > overflow, and then we will need a way to change the LSN anyway. That's just a question of width... Greetings, Andres Freund
