http://www.slony.info/bugzilla/show_bug.cgi?id=235

--- Comment #8 from Christopher Browne <cbbrowne at ca.afilias.info> 2011-08-24 11:56:26 PDT ---
(In reply to comment #7)
> (In reply to comment #5)
> > I agree that the "desired sync time" is confusing and we should move away
> > towards just specifying a max group size.
> 
> Yes, fewer configuration options is better :)  We know the db administrator
> wants to keep the sync lag low.  Having fiddly knobs labeled "make the sync lag
> shorter, except sometimes when it makes them longer" isn't terribly helpful.
> 
> > Does it make more sense to start at max_group_size and reduce that to smaller
> > values on failures?  (this only works if slon doens't get restarted/segfault on
> > the failures)
> 
> The one concern i can see being raised with this approach is that (with a
> radically over-provisioned max_group_size) it's possible for a huge sync group
> to take forever.  However, if there just aren't that many SYNCs available,
> presumably the group would just complete successfully with the set of
> currently-available SYNCs?  In that case, i think the starting-large and
> falling off is better than starting-small and ramping-up.

Yeah, this case concerns me a bit.

The "standard" case where there could be a whole lot of syncs outstanding is
where a subscription is requested, and there's so much data in the set that it
takes multiple days to process it.

At that point, there might be thousands of SYNCs outstanding, and it's possible
that you'll be vulnerable to network glitchiness if slon tries to do the whole
thing in one fell swoop.  

Mind you, the "fallback" of dividing by 2 upon failure should address that,
albeit with some potential for several fairly long failed attempted SYNCs until
the "max" falls to the point where it gets processed OK.

I expect that that's reasonably easily addressed by not setting
sync_group_maxsize super high.

During such a huge subscription, it's not actually terribly useful to have a
huge number of SYNCs; it's not a bad idea to set sync_interval a bit higher at
such times.

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