On Sun, Aug 20, 2023 at 8:40 AM Amit Kapila <amit.kapil...@gmail.com> wrote: > > On Sat, Aug 19, 2023 at 12:46 PM Julien Rouhaud <rjuju...@gmail.com> wrote: > > > > On Sat, 19 Aug 2023, 14:16 Amit Kapila, <amit.kapil...@gmail.com> wrote: > >> > > > >> The idea discussed in the thread [1] is to always persist logical > >> slots to disk during the shutdown checkpoint. I have extracted the > >> patch to achieve the same from that thread and attached it here. This > >> could lead to some overhead during shutdown (checkpoint) if there are > >> many slots but it is probably a one-time work. > >> > >> I couldn't think of better ideas but another possibility is to mark > >> the slot as dirty when we update the confirm_flush LSN (see > >> LogicalConfirmReceivedLocation()). However, that would be a bigger > >> overhead in the running server as it could be a frequent operation and > >> could lead to more writes. > > > > > > Yeah I didn't find any better option either at that time. I still think > > that forcing persistence on shutdown is the best compromise. If we tried to > > always mark the slot as dirty, we would be sure to add regular overhead but > > we would probably end up persisting the slot on disk on shutdown anyway > > most of the time, so I don't think it would be a good compromise. > > > > The other possibility is that we introduce yet another dirty flag for > slots, say dirty_for_shutdown_checkpoint which will be set when we > update confirmed_flush LSN. The flag will be cleared each time we > persist the slot but we won't persist if only this flag is set. We can > then use it during the shutdown checkpoint to decide whether to > persist the slot.
There are already two booleans controlling dirty-ness of slot, dirty and just_dirty. Adding third will created more confusion. Another idea is to record the confirm_flush_lsn at the time of persisting the slot. We can use it in two different ways 1. to mark a slot dirty and persist if the last confirm_flush_lsn when slot was persisted was too far from the current confirm_flush_lsn of the slot. 2. at shutdown checkpoint, persist all the slots which have these two confirm_flush_lsns different. -- Best Wishes, Ashutosh Bapat
