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But does latency matter? Yes, it does, when it comes to single-threaded (synchronous) random reads or writes. Basically, all software that wants the data to be durable does fsync() calls which serialize writes. For example, all DBMSs do. So to understand the performance limit of these apps you should benchmark your cluster with iodepth=1.
The latency doesn’t scale with the number of servers or OSDs-per-SSD or two-RBD-in-RAID0. When you’re benchmarking your cluster with iodepth=1 you’re benchmarking only ONE placement group at a time (PG is a triplet or a pair of OSDs). The result is only affected by how fast a signle single OSD processes a single request. In fact, with iodepth=1 IOPS=1/latency. There is Nick Fisk’s presentation titled «Low-latency Ceph». By «low-latency» he means 0.7ms, which is only ~1500 iops.
=== Micron setup example ===
