Re: Restricting maximum keep segments by repslots

Hello,

At Mon, 6 Nov 2017 05:20:50 -0800, Andres Freund <andres@anarazel.de> wrote in <20171106132050.6apzynxrqrzghb4r@alap3.anarazel.de>
> Hi,
> 
> On 2017-10-31 18:43:10 +0900, Kyotaro HORIGUCHI wrote:
> >   - distance:
> >     how many bytes LSN can advance before the margin defined by
> >     max_slot_wal_keep_size (and wal_keep_segments) is exhasuted,
> >     or how many bytes this slot have lost xlog from restart_lsn.
> 
> 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.

regards,

-- 
Kyotaro Horiguchi
NTT Open Source Software Center