Solr merging hight i/o

We are using Solr with over 30 million records per core,merge factor of 2. When merging/optimizing , the iops increases drastically. Is there any way to regulate the merging 开发者_开发知识库rate without changing merge factor? Like asking solr to use lesser number of threads to do the operations.We can afford slow merging but not such high i/o rate. Found this , any additional information will be very helpful. Thanks