Re: Tracking replication slot "blockings"

On Wed, Apr 16, 2014 at 6:56 PM, Andres Freund <andres(at)2ndquadrant(dot)com>wrote:

> Hi,
>
> On 2014-04-16 18:51:41 +0200, Magnus Hagander wrote:
> > I'm thinking it could be interesting to know how many times (or in some
> > other useful unit than "times" - how often) a specific replication slot
> has
> > "blocked" xlog rotation. Since this AFAIK only happens during
> checkpoints,
> > it seems it should be "reasonably cheap" to track? It would serve as an
> > indicator of which slave(s) are having enough trouble keeping up to
> > potentially cause issues.
>
> The xlog removal code just check the "global minimum" required LSN - it
> doesn't check the individual slots. So you'd need to add a bit more code
> to that location. But it'd be easy.
>

Do we have statistics there somewhere - how often that global minimum
blocks something? That on it's own might be a start :)

But I think I'd just monitor/graph the byte difference for all slots
> using pg_replication_slots...
>

Yeah, that would work when monitored continously. I was more looking for
the view of "hey, could this be what happened" into a system that did not
previously have any monitoring installed and therefor no such history.

--
Magnus Hagander
Me: http://www.hagander.net/
Work: http://www.redpill-linpro.com/