The problem with caching is that if the performance delta between the two storage types isn't large enough, the cost of the caching algorithms and the complexity of managing everything outweigh the performance gains. With Optanes vs. SSDs, the main thing to consider is how busy the devices are in the worst case. Optanes have incredibly low latency and therefore are great at being fast with smaller workloads (as measured by the effective queue depth in iostat) -- but the Optanes I've use typically max out at a queue depth of 15 or so. SSDs aren't as fast at single workoads but the ones I typically use in my ivory tower work better when they are multitasking a lot -- and depending on the type can easily outperform Optanes when there are many things going on at once (in my case the preferred drive is the SN200/SN260 which is excellent at the high queue depth workloads). So the short suggestion is: don't waste time with caching, and analyze your actual workload a bit to be 100% sure where the bottleneck is. Unless you require stupidly low latency and the fastest possible performance for a single user, you will get more bang for your buck by adding more SSDs. If you're not using HDDs, Ceph is usually CPU bound due to limitations in the threading model -- so keep that in mind too (sounds like you already know this if you're doing 4 partitions per Optane :) ). Mark _______________________________________________ ceph-users mailing list ceph-users@xxxxxxxxxxxxxx http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
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