On 2018-04-27 12:28:25 -0400, Bruce Momjian wrote: > On Wed, Apr 18, 2018 at 09:34:50AM -0400, Vick Khera wrote: > > On Wed, Apr 18, 2018 at 12:35 AM, Michael Paquier <mich...@paquier.xyz> > > wrote: > > > > > > That looks like a rather difficult problem to solve in PostgreSQL > > itself, as the operator running the cluster is in charge of setting up > > the FS options which would control the COW behavior, so it seems to me > > > > > > You cannot turn off CoW on ZFS. What other behavior would you refer to here? > > > > I suppose one could make a dedicated data set for the WAL and have ZFS make > > a > > reservation for about 2x the total expected WAL size. It would require > > careful > > attention to detail if you increase WAL segments configuration, though, and > > if > > you had any kind of hiccup with streaming replication that caused the > > segments > > to stick around longer than expected (but that's no different from any other > > file system). > > Uh, at the risk of asking an obvious question, why is the WAL file COW > if it was renamed? No one has the old WAL file open, as far as I know.
Because it's a COW filesystem that doesn't overwrite in place. That's how snapshots etc are implemented. Greetings, Andres Freund
