May 13, 201511 yr Just upgraded to Unraid v6. I understand that it may support dual cache? Currently I have an old 500GB drive functioning as my cache drive. But my Disk Queue Length on my KVMs are pretty large, sometimes 50. However, buying a large SSD gets pricy. I currently have 400GBs of junk on my cache drive. Most of it is files that still need to be organized. My idea is to just add a second cache drive, an SSD to run my KVMs. Is anyone doing this? I was thinking about maybe a 256GB SSD with "Garbage Collection" or Auto Trim. Is anyone doing this?
May 13, 201511 yr I don't think it works that way. Multiple cache drives are done with btrfs raid pools. While it's possible to mix drive sizes and types with btrfs raid, I don't think you get to treat them as separate disks, which is what it sounds like you want to do. Why are you accumulating files on your cache anyway? It is not protected. Move it to the array and organize it later, then you won't need such a large drive. 240GB SSDs are quite reasonable these days.
May 13, 201511 yr you could always just keep the drive outside of the array and use the unassigned devices plugin to mount it. FYI: Currently VM's and Dockers attempt to start BEFORE external drives are mounted, causing the VM's and Dockers to fail. My understanding is that Limetech is working on this in some capacity. The workaround is to start them manually after boot up.
May 13, 201511 yr What it sounds like you are thinking about is dm-cache, which lets you use an SSD as a transparent cache for a slower underlying HDD. You end up with a hybrid drive with tunable parameters. It was mainlined as of 3.9 kernel. Last time I looked at it, it wasn't 100% ready for prime time, but it shows real promise. There is also bcache, which is even more experimental IMHO.
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