Re: [HACKERS] Restricting maximum keep segments by repslots

From: Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>
To: Kyotaro Horiguchi <horikyota(dot)ntt(at)gmail(dot)com>
Cc: jgdr(at)dalibo(dot)com, andres(at)anarazel(dot)de, michael(at)paquier(dot)xyz, sawada(dot)mshk(at)gmail(dot)com, peter(dot)eisentraut(at)2ndquadrant(dot)com, pgsql-hackers(at)lists(dot)postgresql(dot)org, thomas(dot)munro(at)enterprisedb(dot)com, sk(at)zsrv(dot)org, michael(dot)paquier(at)gmail(dot)com
Subject: Re: [HACKERS] Restricting maximum keep segments by repslots
Date: 2020-04-06 16:54:56
Message-ID: 20200406165456.GA4951@alvherre.pgsql
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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(dot)6apzynxrqrzghb4r(at)alap3(dot)anarazel(dot)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

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