I have a small test
cluster (just two nodes) and after rebuilding it several times I
found my latest configuration that SHOULD be the fastest is by far
the slowest (per thread).
I have around 10 spinals that I have an erasure encoded CephFS on. When I installed several SSDs and recreated it with the meta data and the write cache on SSD my performance plummeted from about 10-20MBps to 2-3MBps, but only per thread… I did a rados benchmark and the SSDs Meta and Write pools can sustain anywhere from 50 to 150MBps without issue.
And, if I spool up multiple copies to the FS, each copy adds to that throughput without much of a hit. In fact I can go up to about 8 copied (about 16MBps) before they start slowing down at all. Even while I have several threads actively writing I still benchmark around 25MBps.
Any ideas why single threaded performance would take a hit like this? Almost everything is running on a single node (just a few OSDs on another node) and I have plenty of RAM (96GBs) and CPU (8 Xeon Cores).
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