On 7/30/07, Laurent Raufaste <laurent at over-blog.com> wrote:
>
> Hi,
>
> We are using Slony on a production environment and are very pleased by it.
>
> Our cluster is made of 1 master, 4 slaves that needs to be replicated
> fast, and 2 slaves for which the replication speed isn't a problem.
>
> Here's our issue: In the sl_status view I notice that the st_lag_time is
> always between 1 and many seconds: it goes up to 10 seconds regularly,
> and approximatively one time a day, there is always a slave reaching 1
> min, for example while vacuuming.


1 to 10 seconds is pretty good. If there aren't any changes to tables on the
origin then SYNC events are only generated every 10 seconds (by default). Do
you have enough DML (inserts, updates and deletes) traffic to keep the syncs
flowing? Even if there are plenty of changes to record, by default, syncs
are only generated every 2 seconds, so any lag time of less than 2 seconds
is effectively as up to date as possible.

I tried playing with the folllowing options:
>      -s <milliseconds>     SYNC check interval (default 10000)
>      -t <milliseconds>     SYNC interval timeout (default 60000)
>      -o <milliseconds>     desired subscriber SYNC processing time
>      -g <num>              maximum SYNC group size (default 6)
>
> Now on the master I have:
> -s 1000 -g 50
> On the fast slaves I have:
> -s 1000
> And on the slow slaves:
> -s 10000 -g 10


Are you running slony 1.2? Those defaults look wrong. If you aren't running
1.2.latest then you should consider upgrading.


> I tried lowering the SYNC check interval to 500ms with no real effect,
> and the master is already loaded enough anyway ;)
>
> Is there an effective way to shorten the replication lag time ?


If it's because of inactivity, then you can decrease your
sync_interval_timeout, however the real effect of this is to just make the
lag time look better. (There's an edge case here involving sequences)

If you have plenty of activity then you _might_ consider lowering your
sync_interval (the rate at which slony checks to see if there's anything to
put into a new sync) on your origin. This will increase the load on your
origin, so you will want to do it incrementally and measure the effect.

As your sync interval decreases and load increases, you might need to start
throwing hardware at it. The criteria for deciding that are the same as for
any other database.

Remember that slony is an asynchronous replication system. A system designed
with a strong dependency on very low replication lag time suggests that you
may be trying to fit asynchronous replication into the role of synchronous
replication.

Andrew
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