Re: max_slot_wal_keep_size unit is not specified

At Thu, 25 Nov 2021 11:37:37 -0300, Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org> wrote in
> 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.

(no.. rounded-down X)

> 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.

I thought exactly on that line at development.

Regarding the proposed description, we have never explained how the
same kind of values specified in megabytes is converted into internal
units. If we add that explanation there, we might want the same for
max_wal_size, min_wal_size and wal_keep_size.

> Here's a proposed patch.

+ The value is rounded down to an integer number of WAL files
+ according to <varname>wal_segment_size</varname>.

The same section starts with the following sentences.

> Specify the maximum size of WAL files
> that <link linkend="streaming-replication-slots">replication
> slots</link> are allowed to retain in the <filename>pg_wal</filename>
> directory at checkpoint time.

FWIW I thought (or intended) that "the maximum size of WAL files that
slots are *allowed to retain*" connotes the same to the sentences
cited above.

Premising "16 MB one way or the other don't change things much", I'm
not sure we really need to be particular about a few megabites
difference especially only for this specific variable.

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center