Re: min_safe_lsn column in pg_replication_slots view

Thanks!
At Mon, 6 Jul 2020 20:54:36 -0400, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com> wrote in
> On 2020-Jul-06, Alvaro Herrera wrote:
>
> > Hmm, I like safe_wal_size.

I agree to the name, too.

> > I've been looking at this intermittently since late last week and I
> > intend to get it done in the next couple of days.
>
> 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?

> 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.

> I also made it report null immediately when slots are in state lost.
> But beware of slots that appear lost but fall in the unreserved category
> because they advanced before checkpointer signalled them. (This case
> requires a debugger to hit ...)

Oh! Okay, that change seems right to me.

> 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..)

> Any more thoughts? If not, I'll get this pushed tomorrow finally.

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center