Problem I see is that the RAM cache will not help much as the single plot working space is 290GB, and I think the cache hit are going to be minor as the phases go mostly sequentially through the data. You can split up your array into 2x2 disks though which I suspect would improve on the 1x4. Maybe you could also test plotting 64 buckets as you have the RAM anyways.
Using some RAM as a pure write cache would be beneficial as well
Can you try to break up the hdd-raid? It could be worth a shot to have as mutch hdd-volumes as parallel plots (maybee 10 single-drives, 2 x2 miniraids) and put the ssd-cache in front of it.
if some plots are “random-fetching” blocks from a 14x1 Raid the whole raid-througput should be affekted and the cache-write to disk should slow down.
As differents plots in differents phases of calculation generates different io-patterns separating the raidvolumes + the cache could give you more performance.
I do not think bcache is a good match for Chia workloads. You will be far better off just using those SSD in a raid 0 as a temp drive where you could try plotting up to 3 plots at a time on that, and using the disks not raided as slow but steady 1 plot each plotting temp drives chugging out 14 more plots in parallel.
I’ve got a 720 (with dual 2650 v2s) and 320gb of ram, tried a 270gb ramdrive and plotted one plot.
It still took maybe 16 hours. Certainly doesn’t finish in 1/8th of the time it takes to plot directly to 8 simultaneous sas drive as seperate drives without any SSDs, as I do not want to destroy any hardware in this process. Since I am only throwing 4 threads at this one plot and it seems more isn’t going to improve things then I am not sure what to try next without adding ssds in to the mix. I might try PrimoCache as some people think it helps and allocate a big chunk of my ram to it, this may help all the simultaneous plots. I might also attach maybe 4-8 more USB drives and plot to them simultaneously too, as I am not CPU or ram limited plotting 8 plots at a time. I bought the extra 256GB of ram to see if it would be useful but my conclusion is not really, I had 64GB to begin with which would have been good for 15 parallel plots anyway. 15 plots every 16 hours is nearly 23 plots a day on old recycled hardware and secondhand drives. I’m also using a R515 as a farmer but that can manage about 4-6 plots/day also, and a few old laptops and other old pcs churning a plot or two per day out also.
Sorry for all the edits, its just how my brain works, but I guess if I had a much faster processor and used the ramdrive to plot one plot, using 4 threads from some uber-processor would result in a pretty fast plot, I guess 4 threads with my slow processor just isn’t all that stunning. Its a shame the Chia software doesn’t allow you to throw 32 threads at one plot but then if it did it would become a big power drain like bitcoin, and be all about fast processors rather than having a lot of drives.