Add dm-cache documentation for XOSTOR

[Wescoeur]

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[olivierlambert]

I'm not sure to understand the cache system: are reads already "cached" by design in LINSTOR? ie doesn't wait to read to another node but having the local disk speed for it. Last time I did a bench, this is what I had. So I am probably ignoring something obvious in there.

[Wescoeur]

DRBD can use the Linux system's buffers. But by design you don't have a cache: the read requests can be sent to the local host currently used and/or on another machine to accelerate parallel requests.

The interest of dm-cache here is to use HDDs (or slow SSDs) on the DRBD level and a faster device like an NVME to cache the most read blocks.