Re: needless complexity in StartupXLOG - Mailing list pgsql-hackers

At Mon, 26 Jul 2021 16:15:23 -0400, Stephen Frost <sfrost@snowman.net> wrote in 
> Greetings,
> 
> * Robert Haas (robertmhaas@gmail.com) wrote:
> > On Mon, Jul 26, 2021 at 1:32 PM Stephen Frost <sfrost@snowman.net> wrote:
> > > Yeah, tend to agree with this too ... but something I find a bit curious
> > > is the comment:
> > >
> > > * Insert a special WAL record to mark the end of
> > > * recovery, since we aren't doing a checkpoint.
> > >
> > > ... immediately after setting promoted = true, and then at the end of
> > > StartupXLOG() having:
> > >
> > > if (promoted)
> > >         RequestCheckpoint(CHECKPOINT_FORCE);
> > >
> > > maybe I'm missing something, but seems like that comment isn't being
> > > terribly clear.  Perhaps we aren't doing a full checkpoint *there*, but
> > > sure looks like we're going to do one moments later regardless of
> > > anything else since we've set promoted to true..?
> > 
> > Yep. So it's a question of whether we allow operations that might
> > write WAL in the meantime. When we write the checkpoint record right
> > here, there can't be any WAL from the new server lifetime until the
> > checkpoint completes. When we write an end-of-recovery record, there
> > can. And there could actually be quite a bit, because if we do the
> > checkpoint right in this section of code, it will be a fast
> > checkpoint, whereas in the code you quoted above, it's a spread
> > checkpoint, which takes a lot longer. So the question is whether it's
> > reasonable to give the checkpoint some time to complete or whether it
> > needs to be completed right now.
> 
> All I was really trying to point out above was that the comment might be
> improved upon, just so someone understands that we aren't doing a
> checkpoint at this particular place, but one will be done later due to
> the promotion.  Maybe I'm being a bit extra with that, but that was my
> thought when reading the code and the use of the promoted flag variable.

(I feel we don't need to check for last-checkpoint, too.)

FWIW, by the way, I complained that the variable name "promoted" is a
bit confusing.  It's old name was fast_promoted, which means that fast
promotion is being *requsted* and ongoing.  On the other hand the
current name "promoted" still means "(fast=non-fallback) promotion is
ongoing" so there was a conversation as the follows.

https://www.postgresql.org/message-id/9fdd994d-a531-a52b-7906-e1cc22701310%40oss.nttdata.com

>> How about changing it to fallback_promotion, or some names with more
>> behavior-specific name like immediate_checkpoint_needed?
> 
> I like doEndOfRecoveryCkpt or something, but I have no strong opinion
> about that flag naming. So I'm ok with immediate_checkpoint_needed
> if others also like that, too.

regards.

-- 
Kyotaro Horiguchi
NTT Open Source Software Center