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 is 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, then 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 and can be set next.