Re: [HACKERS] Restricting maximum keep segments by repslots

At Tue, 31 Mar 2020 16:59:05 -0300, Alvaro Herrera <alvherre@2ndquadrant.com> wrote in 
> On 2020-Mar-31, Alvaro Herrera wrote:
> 
> > On 2020-Mar-31, Alvaro Herrera wrote:
> > 
> > > I'm not sure if I explained my proposal clearly.  What if
> > > XLogGetLastRemovedSegno returning zero means that every segment is
> > > valid?  We don't need to scan pg_xlog at all.
> > 
> > I mean this:
> 
> [v21 does it that way.  Your typo fixes are included, but not the
> LastRemoved stuff being discussed here.  I also edited the shortdesc in
> guc.c to better match {min,max}_wal_size.]
> 
> Hmm ... but if the user runs pg_resetwal to remove WAL segments, then
> this will work badly for a time (until a segment is removed next).  I'm
> not very worried for that scenario, since surely the user will have to
> reclone any standbys anyway.  I think your v20 behaves better in that
> case.  But I'm not sure we should have that code to cater only to that
> case ... seems to me that it will go untested 99.999% of the time.

I feel the same. If we allow bogus status or "unkown" status before
the first checkpoint, we don't need to scan the directory.

> Maybe you're aware of some other cases where lastRemovedSegNo is not
> correct for the purposes of this feature?

The cases of archive-failure (false "removed") and change of
max_slot_wal_keep_size(false "normal/kept") mentioned in another mail.

> I pushed the silly test_decoding test adjustment to get it out of the
> way.
> 
> /me tries to figure out KeepLogSeg next

Thanks.

regards.

-- 
Kyotaro Horiguchi
NTT Open Source Software Center