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# Ceph isn’t slow on HDDs: theoretical single-thread random write performance of Bluestore is 66 % (2/3) of your drive’s IOPS (currently it’s 33 % in practice, but if you push this handbrake down: https://github.com/ceph/ceph/pull/26909 it goes back to 66 %), and multi-threaded read/write performance is about 100 % of the raw drive speed.
'''However''', the naive expectation is that as you replace your HDDs to with SSDs and use a fast network — Ceph should become almost as faster. Everyone is used to the idea that I/O is slow and software is fast. And this is generally NOT true with Ceph.
Ceph is a Software-Defined Storage system, and its «software» is a significant overhead. The general rule currently is: with Ceph it’s hard to achieve random read latencies less than 0.5ms and random write latencies less than 1ms, '''no matter what drives or network you use'''. This stands for only 2000 iops random read and 1000 iops random write with one thread, and even if you manage to achieve this result you’re already in a good shape. With BIS hardware and some tuning you may be able to improve it further, but only twice or so.
