Hi, On 2022-07-11 15:08:57 -0400, Robert Haas wrote: > On Mon, Jul 11, 2022 at 2:57 PM Andres Freund <and...@anarazel.de> wrote: > > I don't know where we could fit a sanity check that connects to all > > databases > > and detects duplicates across all the pg_class instances. Perhaps > > pg_amcheck? > > Unless we're going to change the way CREATE DATABASE works, uniqueness > across databases is not guaranteed.
You could likely address that by not flagging conflicts iff oid also matches? Not sure if worth it, but ... > > Maybe the easiest fix here would be to replace the file atomically. Then we > > don't need this <= 512 byte stuff. These are done rarely enough that I don't > > think the overhead of creating a separate file, fsyncing that, renaming, > > fsyncing, would be a problem? > > Anything we can reasonably do to reduce the number of places where > we're relying on things being <= 512 bytes seems like a step in the > right direction to me. It's very difficult to know whether such code > is correct, or what the probability is that crossing the 512-byte > boundary would break anything. Seems pretty simple to do. Have write_relmapper_file() write to a .tmp file first (likely adding O_TRUNC to flags), use durable_rename() to rename it into place. The tempfile should probably be written out before the XLogInsert(), the durable_rename() after, although I think it'd also be correct to more closely approximate the current sequence. It's a lot more problematic to do this for the control file, because we can end up updating that at a high frequency on standbys, due to minRecoveryPoint. I have wondered about maintaining that in a dedicated file instead, and perhaps even doing so on a primary. Greetings, Andres Freund
