Since kernel 4.0, the RBD driver uses blk-mq, which is designed to offer higher performance than the older queuing system. By default, the maximum outstanding requests possible with RBD when using blk-mq are 128. For most use cases, this is more than enough; however, if your workload needs to utilize the full power of a large Ceph cluster, you may find that only having 128 outstanding requests is not enough. There is an option that can be specified when mapping the RBD to increase this value, which can be set next.