the default Serverconf file does not seem to have oplogSize setting at all. So how is the 1Gig limit enforced?
oplogSize = <integer>
* The size of the replication operation log, in MB, for environments
with search head clustering or search head pooling.
In a standalone environment, 20% of this size is used.
* After the KV Store has created the oplog for the first time, changing this
setting does NOT affect the size of the oplog. A full backup and restart
of the KV Store is required.
* Do not change this setting without first consulting with Splunk Support.
* Default: 1000MB (1GB)
The default is active regardless. You should contact support on the correct steps on increasing opLog in a search head cluster. There is a very very specific order you have to do things to not wipe out your kvstore contents and it can be done without a backup and restore. I would recommend 10GB in an active ES environment.