this is getting absurd. I think we all know the relative prices and performances of off-the-shelf disks/ssd/ram. each have peculiarities that make their use somewhat complex.
- with disks, you have to think about seek time, since it can range from zero to ~15ms. for some workloads, a saving grace is that with request sorting (helped also by disk-level queue reordering), you can perform several transactions along the way. disks have historically followed a pretty steep curve of improved density, somewhat akin to Moore's law, which has delivered ever-higher density and attendant bandwidth. - with ram, you get the proverbial random access. like most proverbs, that's only a little true: ram has banking and paging effects, and emphatically rewards sequential access. it also suffers from a very stiff industry that hasn't managed to adopt a transactional interface, even though cpus have evermore internal concurrency. (caches have let dram designers stay very lazy...) - with flash, you'll probably never have a random-access interface - it'll always be a disk-like block-transfer thing. why? because flash has to be remapped to be useful, and that remapping has to change during use (indeed, *because*of* use). the discussion of PCIe and NVMe were pretty much a diversion, since none of them are substantially altering the block-transfer nature of flash. yes, something like NVMe does simplify the protocol being employed, but it's still a mechanism for queueing block-transfer requests, like any IO device (SATA, SCSI, RAID, even eth/IB networks.) it would be amusing to see a flash vendor take a page from networks, and offer "flash rdma". but frankly, I'm not sure there's enough niche for high-end flash at all. high-volume devices will always just follow the capacity-performance bounds of current flash fabs. an awfully big chunk of the IT industry wants *distributed* performance (the Googles of the world) and won't normally want to pay the premium of a PCIe flash device, since they can get arbitrary aggregate performance with big clusters, and need big clusters anyway for other reasons. _______________________________________________ Beowulf mailing list, Beowulf@beowulf.org sponsored by Penguin Computing To change your subscription (digest mode or unsubscribe) visit http://www.beowulf.org/mailman/listinfo/beowulf
