Situation: I need to use EC pools (for the economics/power/cooling) for the storage of data, but my use case requires a block device. Ergo, I require a cache tier. I have tried using a 3x replicated pool as a cache tier - the throughput was poor, mostly due to latency, mostly due to device saturation (i.e., of the tier devices), mostly due to seeking.
Data on the cache tier is tactical: it's going to get pushed off the cache tier into the EC pool relatively quickly. RAID-6 protection (which is roughly the same as I
get with a 3x replicated pool) is fine.
I happen to have the ability to create a small RAID-6 (on each of several nodes) that could collectively serve as a cache tier. And the RAID controller has a battery, so can operate write-back, so latency goes way down. Can I create a pool of unreplicated OSDs, i.e., can I set the size of the pool to 1? It seems like this creates a singularity when it comes to CRUSH: do placement groups even make sense? Or is there any way that I can use my RAID hardware to build a low-latency cache tier?
Regards,
-don-
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