Re: Load Distributed Checkpoints, final patch

Tom Lane wrote:
> Heikki Linnakangas <heikki@enterprisedb.com> writes:
>> We could just allow any value up to 1.0, and note in the docs that you
>> should leave some headroom, unless you don't mind starting the next
>> checkpoint a bit late. That actually sounds pretty good.
>
> Yeah, that sounds fine.  There isn't actually any harm in starting a
> checkpoint later than otherwise expected, is there?  The worst
> consequence I can think of is a backend having to take time to
> manufacture a new xlog segment, because we didn't finish a checkpoint
> in time to recycle old ones.  This might be better handled by allowing
> a bit more slop in the number of recycled-into-the-future xlog segments.
>
> Come to think of it, shouldn't we be allowing some extra slop in the
> number of future segments to account for xlog archiving delays, when
> that's enabled?

XLogFileSlop is currently 2 * checkpoint_segments + 1 since last
checkpoint, which is just enough to accommodate a checkpoint that lasts
the full checkpoint interval. If we want to keep as much "slop" there as
before, then yes that should be increased to (2 +
checkpoint_completion_target) * checkpoint_segments + 1, or just 3 *
checkpoint_segments if we want to keep it simple.

--
   Heikki Linnakangas
   EnterpriseDB   http://www.enterprisedb.com