void createOplog(OperationContext* opCtx, const std::string& oplogCollectionName, bool isReplSet) {
 Lock::GlobalWrite lk(opCtx);

const ReplSettings& replSettings = ReplicationCoordinator::get(opCtx)->getSettings();

OldClientContext ctx(opCtx, oplogCollectionName);
 Collection* collection = ctx.db()->getCollection(opCtx, oplogCollectionName);

if (collection) {
 if (replSettings.getOplogSizeBytes() != 0) {
 const CollectionOptions oplogOpts =
 collection->getCatalogEntry()->getCollectionOptions(opCtx);

int o = (int)(oplogOpts.cappedSize / (1024 * 1024));
 int n = (int)(replSettings.getOplogSizeBytes() / (1024 * 1024));
 if (n != o)

{ stringstream ss; ss << "cmdline oplogsize (" << n << ") different than existing (" << o << ") see: http://dochub.mongodb.org/core/increase-oplog"; log() << ss.str() << endl; uasserted(13257, ss.str()); }

}