On Thursday IBM announced its X6 product family, the sixth generation of the company’s successful EXA server architecture. A smaller byline of the introduction was the company’s new eXFlash memory-channel storage or eXFlash DIMM which is offered as one of many flash options available to X6 users.
Close followers of The SSD Guy already know that I am a serious advocate of putting flash onto the memory bus. Why slow the technology down by hiding it behind an interface that was optimized for slower rotating media? SATA and SAS are very slow ways of delivering fast data.
Putting flash on the PCIe bus, which was originally designed as a means of adding coprocessors to a system, is a good step in the right direction, but it still isn’t the optimum solution. Adding flash to the fastest bus in the system removes further delays and frees the flash to perform at its peak. The only improvement on this solution would be to add a flash-optimized bus to the processor, a step that has not yet been taken, but that I expect to evolve as the industry slowly awakens to the notion that flash has a role in all computers.
To that end IBM conceived an initiative to put flash DIMMs onto its DRAM bus, something the company calls “memory-channel storage.” The company teamed up with SanDisk (formerly SMART) and Diablo to create the product that SanDisk calls the ULLtra DIMM, or “Ultra Low Latency” flash DIMM, which was introduced by these companies last summer. Now that IBM has validated the technology in its own systems it will make flash DIMMs available to its customers, sampling today, with general availability slated for April.
IBM tells me that the eXFlash DIMM starts with the ULLtra DIMM and adds proprietary techniques to tighten its write latency down to a mere 3.3µs. Latency reduction is a big part of IBM’s flash initiative. The company says its eXFlash DIMM has up to 83% lower latency than a PCIe flash drive and 95% lower latency than an SSD. It provides 125K read and 65Kwrite IOPS consistently under mixed workloads, performance that scales in proportion to the number of DIMMs added to the system. (PCIe SSDs have some trouble scaling performance to the number of units due to bus contention.) IBM has told me that X6 systems have been qualified with as many as 32 eXFlash DIMMs (out of 96 maximum DIMM slots per system).
The eXFlash DIMM uses a standard DDR3 interface and comes in 200GB and 400GB capacities. It is fully supported by IBM’s FlashCache Storage Accelerator software and is configurable as a block device or as cache storage.