On Mon, Sep 16, 2024 at 10:41 PM Bharath Rupireddy <bharath.rupireddyforpostg...@gmail.com> wrote: > > Thanks for looking into this. > > On Mon, Sep 16, 2024 at 4:54 PM Amit Kapila <amit.kapil...@gmail.com> wrote: > > > > Why raise the ERROR just for timeout invalidation here and why not if > > the slot is invalidated for other reasons? This raises the question of > > what happens before this patch if the invalid slot is used from places > > where we call ReplicationSlotAcquire(). I did a brief code analysis > > and found that for StartLogicalReplication(), even if the error won't > > occur in ReplicationSlotAcquire(), it would have been caught in > > CreateDecodingContext(). I think that is where we should also add this > > new error. Similarly, pg_logical_slot_get_changes_guts() and other > > logical replication functions should be calling > > CreateDecodingContext() which can raise the new ERROR. I am not sure > > about how the invalid slots are handled during physical replication, > > please check the behavior of that before this patch. > > When physical slots are invalidated due to wal_removed reason, the failure > happens at a much later point for the streaming standbys while reading the > requested WAL files like the following: > > 2024-09-16 16:29:52.416 UTC [876059] FATAL: could not receive data from WAL > stream: ERROR: requested WAL segment 000000010000000000000005 has already > been removed > 2024-09-16 16:29:52.416 UTC [872418] LOG: waiting for WAL to become > available at 0/5002000 > > At this point, despite the slot being invalidated, its wal_status can still > come back to 'unreserved' even from 'lost', and the standby can catch up if > removed WAL files are copied either by manually or by a tool/script to the > primary's pg_wal directory. IOW, the physical slots invalidated due to > wal_removed are *somehow* recoverable unlike the logical slots. > > IIUC, the invalidation of a slot implies that it is not guaranteed to hold > any resources like WAL and XMINs. Does it also imply that the slot must be > unusable? >
If we can't hold the dead rows against xmin of the invalid slot, then how can we make it usable even after copying the required WAL? -- With Regards, Amit Kapila.
