Date: Mon, 6 Apr 2020 12:54:56 -0400
From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Cc: jgdr@dalibo.com, andres@anarazel.de, michael@paquier.xyz,
	sawada.mshk@gmail.com, peter.eisentraut@2ndquadrant.com,
	pgsql-hackers@lists.postgresql.org, thomas.munro@enterprisedb.com,
	sk@zsrv.org, michael.paquier@gmail.com
Subject: Re: [HACKERS] Restricting maximum keep segments by repslots
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On 2020-Apr-06, Kyotaro Horiguchi wrote:

> > * Andres complained that the "distance" column was not a great value to
> >   expose (20171106132050.6apzynxrqrzghb4r@alap3.anarazel.de).  That's
> >   right: it changes both by the insertion LSN as well as the slot's
> >   consumption.  Maybe we can expose the earliest live LSN (start of the
> >   earliest segment?) as a new column.  It'll be the same for all slots,
> >   I suppose, but we don't care, do we?
> 
> I don't care as far as users can calculate the "remain" of individual
> slots (that is, how far the current LSN can advance before the slot
> loses data). But the "earliest live LSN (EL-LSN) is really not
> relevant to the safeness of each slot. The distance from EL-LSN to
> restart_lsn or the current LSN doesn't generally suggest the safeness
> of individual slots.  The only relevance would be if the distance from
> EL-LSN to the current LSN is close to max_slot_wal_keep_size, the most
> lagged slot could die in a short term.

Thanks for the revised version.  Please note that you forgot to "git
add" the test file, to it's not in the patch.

I'm reviewing the patch now.

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services