Question

We have an application that uses MongoDB for log keeping. We run a pair of machines in replica set. One machine is used for writes, another for reads (reports and lookups).

By looking at MongoDB docs I can see that there is a “buildIndexes” config option for replica set.
Is there a downside of disabling indexes on a machine that is only used for writes? Would I need to “lock” a machine in a replica set to be always Master?

http://www.mongodb.org/display/DOCS/Replica+Set+Configuration

Answer 1

It seems that you need set buildIndexes option to false for write machine:

When false, prevent secondary indexes
from being created on this member.
This is typically used on machines
that are pure “backup” machines that
are never queried. By not having the
secondary indexes, the member performs
less works on writes and requires less
ram. Note the _id index is still
created. Can only be set to false if
priority:0. It is rare to use this
option.

But in general i guess that for reports you need option to build indexes in background for both machines. But seems that mongodb doesn’t support background index building well for replica set.

RavenDB build indexes in background by default (only a week ago was added option to build in sync ;)). I guess background indexes best solution for reports.