How to Make Flash Accessible on the Memory Bus

One of the more interesting technologies at the Flash Memory Summit this year was presented by SMART Storage and Diablo Technologies. They’ve taken server-side flash implementation to a new level with SMART Storage’s ULLtraDIMM, a flash storage device that puts NAND flash capacity on the memory bus. These 400GB and 800GB capacity DDR3 form-factor DIMM modules make flash capacity available to the CPU as either a DRAM memory expansion or as block storage, like a PCIe flash solution.

Diablo Technologies, a memory technology company, developed Memory Channel Storage (MCS) that enables flash on a DIMM module to be accessed by the CPU, instead of using the SATA bus as other DIMM form-factor SSD products have. Using a host-level driver and an ASIC on the DIMM, it creates a special memory storage layer in flash through which the CPU actually moves data from the RAM memory space. It also requires a minor modification in the server BIOS to be supported by the CPU, something that three OEMs have currently completed.

Each DIMM flash module has 16 separate data channels, which are independently addressable. This enables parallel data writes by the driver, improving performance over the DMA process used by PCIe-based solutions. Specs for ULLtraDIMM are showing 5 microsecond latencies for these devices, an order of magnitude better than typical PCIe flash products. This architecture also enables up to 63 ULLtraDIMM modules to be aggregated creating 25TB of flash capacity and >9M IOPS in a single server.

The driver takes writes and coalesces them into larger 4KB blocks of data that are compatible with the page-level writes that flash expects. ULLtraDIMM uses SMART Modular’s Guardian technology to manage endurance and reliability of the 19nm MLC flash. It has capacitor-backed DRAM and full data path protection creating an MTBF of 2 million hours, with published specs of 10 drive writes per day for 5 years.

The benefits for this technology are reducing latency in server-side flash devices and increasing flash capacity, as most low profile servers have more DIMM slots available than they do PCIe or SAS/SATA slots. With this solution, ultra high performance server configurations can be built using these low latency flash modules. As memory expansion it can provide TBs of memory capacity using flash at a fraction of the cost of DRAM.

Storage Swiss Take

Flash solutions were originally implemented as high-performance hard disk replacements. While some flash companies pushed the concept of SSDs as ‘storage memory’ instead of fast disk drives, flash wasn’t really replacing DRAM. Now, SMART Storage and Diablo Technology have given this concept some real substance by creating a solution for the server-side flash market that puts flash on the memory bus and allows it to be accessed like DRAM. The result is a disruptive solution that can make large memory implementations affordable and dramatically increases the capacity and performance of server-side flash.

Eric is an Analyst with Storage Switzerland and has over 25 years experience in high-technology industries. He’s held technical, management and marketing positions in the computer storage, instrumentation, digital imaging and test equipment fields. He has spent the past 15 years in the data storage field, with storage hardware manufacturers and as a national storage integrator, designing and implementing open systems storage solutions for companies in the Western United States.  Eric earned degrees in electrical/computer engineering from the University of Colorado and marketing from California State University, Humboldt.  He and his wife live in Colorado and have twins in college.

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