On Mon, Jun 22, 2020 at 6:32 PM Fujii Masao <masao.fu...@oss.nttdata.com> wrote: > > On 2020/06/22 21:01, Amit Kapila wrote: > > On Mon, Jun 22, 2020 at 11:19 AM Michael Paquier <mich...@paquier.xyz> > > wrote: > >> > >> On Sat, Jun 20, 2020 at 03:53:54PM +0900, Michael Paquier wrote: > >>> On Sat, Jun 20, 2020 at 09:45:52AM +0530, Amit Kapila wrote: > >>>> Isn't this information specific to checkpoints, so maybe better to > >>>> display in view pg_stat_bgwriter? > >>> > >>> Not sure that's a good match. If we decide to expose that, a separate > >>> function returning a LSN based on the segment number from > >>> XLogGetLastRemovedSegno() sounds fine to me, like > >>> pg_wal_last_recycled_lsn(). Perhaps somebody has a better name in > >>> mind? > >> > >> I was thinking on this one for the last couple of days, and came up > >> with the name pg_wal_oldest_lsn(), as per the attached, traking the > >> oldest WAL location still available. > > Thanks for the patch! > > + <literal>NULL</literal> if no WAL segments have been removed since > + startup. > > Isn't this confusing? I think that we should store the last removed > WAL segment to somewhere (e.g., pg_control) and restore it at > the startup, so that we can see the actual value even after the startup. > Or we should scan pg_wal directory and find the "minimal" WAL segment > and return its LSN. > > > > I feel such a function is good to have but I am not sure if there is a > > need to tie it with the removal of min_safe_lsn column. > > We should expose the LSN calculated from > "the current WAL LSN - max(wal_keep_segments * 16MB, max_slot_wal_keep_size)"? > This indicates the minimum LSN of WAL files that are guaraneed to be > currently retained by wal_keep_segments and max_slot_wal_keep_size. > That is, if checkpoint occurs when restart_lsn of replication slot is > smaller than that minimum LSN, some required WAL files may be removed. > > So DBAs can periodically monitor and compare restart_lsn and that minimum > LSN. If they see frequently that difference of those LSN is very small, > they can decide to increase wal_keep_segments or max_slot_wal_keep_size, > to prevent required WAL files from being removed. Thought? >
+1. This sounds like a good and useful stat for users. -- With Regards, Amit Kapila. EnterpriseDB: http://www.enterprisedb.com
