Re: Restricting maximum keep segments by repslots

Oops! The previous patch is forgetting the default case and crashes.

At Wed, 08 Nov 2017 13:14:31 +0900 (Tokyo Standard Time), Kyotaro HORIGUCHI <horiguchi.kyotaro@lab.ntt.co.jp> wrote in <20171108.131431.170534842.horiguchi.kyotaro@lab.ntt.co.jp>
> > I don't think 'distance' is a good metric - that's going to continually
> > change. Why not store the LSN that's available and provide a function
> > that computes this? Or just rely on the lsn - lsn operator?
> 
> It seems reasonable.,The 'secured minimum LSN' is common among
> all slots so showing it in the view may look a bit stupid but I
> don't find another suitable place for it.  distance = 0 meant the
> state that the slot is living but insecured in the previous patch
> and that information is lost by changing 'distance' to
> 'min_secure_lsn'.
> 
> Thus I changed the 'live' column to 'status' and show that staus
> in text representation.
> 
> status: secured | insecured | broken
> 
> So this looks like the following (max_slot_wal_keep_size = 8MB,
> which is a half of the default segment size)
> 
> -- slots that required WAL is surely available
> select restart_lsn, status, min_secure_lsn, pg_current_wal_lsn() from pg_replication_slots;
> restart_lsn | status  | min_recure_lsn | pg_current_wal_lsn 
> ------------+---------+----------------+--------------------
> 0/1A000060  | secured | 0/1A000000     | 0/1B42BC78
> 
> -- slots that required WAL is still available but insecured
> restart_lsn | status    | min_recure_lsn | pg_current_wal_lsn 
> ------------+-----------+----------------+--------------------
> 0/1A000060  | insecured | 0/1C000000     | 0/1D76C948
> 
> -- slots that required WAL is lost
> # We should have seen the log 'Some replication slots have lost...'
> 
> restart_lsn | status | min_recure_lsn | pg_current_wal_lsn 
> ------------+--------+----------------+--------------------
> 0/1A000060  | broken | 0/1C000000     | 0/1D76C9F0
> 
> 
> I noticed that I abandoned the segment fragment of
> max_slot_wal_keep_size in calculating in the routines. The
> current patch honors the frament part of max_slot_wal_keep_size.

I changed IsLsnStillAvailable to return meaningful values
regardless whether max_slot_wal_keep_size is set or not.

# I had been forgetting to count the version for latestst several
# patches. I give the version '4' - as the next of the last
# numbered patch.

-- 
Kyotaro Horiguchi
NTT Open Source Software Center