Date: Mon, 6 Jul 2020 23:01:33 -0400
From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Cc: amit.kapila16@gmail.com, masao.fujii@oss.nttdata.com,
	michael@paquier.xyz, pgsql-hackers@lists.postgresql.org
Subject: Re: min_safe_lsn column in pg_replication_slots view
Message-ID: <20200707030133.GA29468@alvherre.pgsql>
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On 2020-Jul-07, Kyotaro Horiguchi wrote:

> At Mon, 6 Jul 2020 20:54:36 -0400, Alvaro Herrera <alvherre@2ndquadrant.com> wrote in 

> > I propose the attached.  This is pretty much what was proposed by
> > Kyotaro, but I made a couple of changes.  Most notably, I moved the
> > calculation to the view code itself rather than creating a function in
> > xlog.c, mostly because it seemed to me that the new function was
> > creating an abstraction leakage without adding any value; also, if we
> > add per-slot size limits later, it would get worse.
> 
> I'm not sure that detailed WAL segment calculation fits slotfuncs.c
> but I don't object to the change.  However if we do that:
> 
> +			/* determine how many segments slots can be kept by slots ... */
> +			keepSegs = max_slot_wal_keep_size_mb / (wal_segment_size / (1024 * 1024));
> 
> Couldn't we move ConvertToXSegs from xlog.c to xlog_ingernals.h and
> use it intead of the bare expression?

I was of two minds about that, and the only reason I didn't do it is
that we'll need to give it a better name if we do it ...  I'm open to
suggestions.

> > The other change was to report negative values when the slot becomes
> > unreserved, rather than zero.  It shows how much beyond safety your
> > slots are getting, so it seems useful.  Clamping at zero seems to serve
> > no purpose.
> 
> The reason for the clamping is the signedness of the values, or
> integral promotion.  However, I believe the calculation cannot go
> beyond the range of signed long so the signedness conversion in the
> patch looks fine.

Yeah, I think the negative values are useful to see.  I think if you
ever get close to 2^62, you're in much more serious trouble anyway :-)
But I don't deny that the math there could be subject of overflow
issues.  If you want to verify, please be my guest ...

> > One thing that got my attention while going over this is that the error
> > message we throw when making a slot invalid is not very helpful; it
> > doesn't say what the current insertion LSN was at that point.  Maybe we
> > should add that?  (As a separate patch, of couse.)
> 
> It sounds helpful to me. (I remember that I sometime want to see
> checkpoint LSNs in server log..)

Hmm, ... let's do that for pg14!

Thanks for looking,

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