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Feeding the XPoint cuckoo and finding it a place in the storage nest
Intel/Micron tech looks to force its way into the memory storage spectrum
XPoint memory is a spectrum-invading cuckoo. Spectrums are great ways to describe a range including different items with differing characteristics. Everyone gets used to the range, then something comes along and forces its way into the spectrum, making existing items shuffle up and down.
There is a memory-storage spectrum with high-speed, high-cost and low-capacity DRAM at one end, and slow, cheap, capacious tape at the other. They form a hierarchy, ordered by speed, cost and capacity. A diagram can show the DRAM, NAND, disk and tape elements on this spectrum:
Memory storage spectrum or hierarchy
We show developing NVMe NAND sitting between DRAM and SSD NAND using SAS and SATA interfaces. Such NVMe NAND could be treated as storage-class memory, logically existing in the same address space as DRAM, if host OS/hypervisor and possibly application software changes are made to support its use.
EMC's developing DSSD flash array, accessed across an NVMe fabric, is technology in this area.
There is also mention of Diablo Technologies' Memory Channel flashDIMMs too, since Lenovo and two other server manufacturers support its use.
The diagram also shows SAS/SATA NAND invading the 15,000rpm disk drive part of the spectrum.
Intel/Micron's 3D XPoint memory will slot in between NVME NAND and DRAM, and the new memory-storage hierarchy will look like this:
- Storage-class XPoint memory
- NVMe XPoint memory (meaning no host/app software changes needed)
- NVMe flash
- SAS/SATA SSDs
- Disk drives
- Optical media (to be pernickety)
Here's the diagram refreshed to show XPoint; we've not included optical storage as we don't consider it to be a mainstream technology.
New memory-storage spectrum with XPoint memory
FlashDIMMS are shown in the same region as XPoint. Until we know actual XPoint speeds, we can't be any more precise than that. SAS and SATA SSDs are pushing into the 10,000rpm disk drive space, with 3D TLC (3bits/cell) NAND being at the forefront of this.
With this new hierarchy in mind, let's have a punt at seeing how XPoint memory might actually be used.
XPoint memory is up to 1,000 times faster than NAND, but it can barely reach 20 per cent of the speeds DRAM is capable of. This may mean that a SATA-connected XPoint SSD will have a large proportion of its access latency taken up by the SATA interface, and also that current state-of-the-art 12Gbit/s SATA restricts its bandwidth.
This could apply to SAS as well, meaning that for practical purposes, we will only see XPoint memory with NVMe PCIe interfaces, or fitted in DIMM slots.
XPoint has the same storage density as 2D planar NAND but is ten times denser than DRAM, and, obviously, will not be as dense as 3D NAND.
It will not cost as much as DRAM but will cost more than NAND. Actual prices don't exist yet and we have not seen relative cost multiples, such as XPoint will be five times more costly than NAND, but half the price of DRAM.
Intel and Micron say XPoint memory is complementary to the DRAM and NAND markets. Micron says, for example, that it won't affect its 3D NAND developments. But if XPoint memory is not meant to affect the DRAM and 3D NAND markets, then it has to develop a new market where neither of these technologies fit.
Intel and Micron say new applications in which XPoint is used as storage-class memory will need new computer architecture designs and software changes.