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 waydown. 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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