Re: Allow "snapshot too old" error, to prevent bloat

On Wed, Feb 18, 2015 at 10:57 PM, Kevin Grittner <kgrittn(at)ymail(dot)com> wrote:

> Magnus Hagander <magnus(at)hagander(dot)net> wrote:
> > On Feb 17, 2015 12:26 AM, "Andres Freund" <andres(at)2ndquadrant(dot)com>
> wrote:
> >> On 2015-02-16 16:35:46 -0500, Bruce Momjian wrote:
> >> But max_standby_streaming_delay, max_standby_archive_delay and
> >> hot_standby_feedback are among the most frequent triggers for
> >> questions and complaints that I/we see.
> >>
> > Agreed.
> > And a really bad one used to be vacuum_defer_cleanup_age, because
> > of confusing units amongst other things. Which in terms seems
> > fairly close to Kevins suggestions, unfortunately.
>
> Particularly my initial suggestion, which was to base snapshot
> "age" it on the number of transaction IDs assigned. Does this look
> any better to you if it is something that can be set to '20min' or
> '1h'? Just to restate, that would not automatically cancel the
> snapshots past that age; it would allow vacuum of any tuples which
> became "dead" that long ago, and would cause a "snapshot too old"
> message for any read of a page modified more than that long ago
> using a snapshot which was older than that.
>

Yes, it would definitely look much better. My reference per above was
exactly that - having a setting in the unit "number of xids" confused a lot
of users and made it really hard to tune. Having something in time units is
a lot easier to understand and tune for most people.

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