Re: max_slot_wal_keep_size unit is not specified

On 2021-Nov-24, Kyotaro Horiguchi wrote:

> At Tue, 23 Nov 2021 11:21:05 -0300, Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org> wrote in

> > I wonder why did we make it round down rather than up. Does this mean
> > that if you have max_slot_wal_keep_size=8MB and wal segments of 16 MB,
> > the slot gets invalidated with more than zero reserved segments?
>
> That rounding behavior is the way we are generally doing on similar
> variables. It is based on the behavior of ConvertToXSegs(). So
> max_wal_size for example is rounded-up the same way.

After thinking on it some more, I think rounding down is correct. If
somebody sets max_slot_wal_keep_size=40MB, then by keeping the limit at
32 MB (round down) we're honoring that request better than if we made it
48 MB (round up). But in any case, I think it doesn't matter much:
users need a *rough* size limit, not a super-precise one. After all,
this is only verified at checkpoint time. We're probably dealing with a
few hundreds of megabytes at least, so 16 MB one way or the other don't
change things much.

Here's a proposed patch.

--
Álvaro Herrera 39°49'30"S 73°17'W — https://www.EnterpriseDB.com/
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