WD is sampling SMR – Shingled Magnetic Recording – disk drives to several cloud vendors including Facebook, Google, Microsoft and Amazon. Seagate announced sales of a million SMR drives in September. HGST likely has a similar program under way.
SMR in a minute
Disk drive tracks are separated by a gap. In SMR drives, this gap is removed to roughly double density, says a paper from CMU’s Parallel Data Lab.
Write heads lay down a wider track than read heads need. In an SMR disk the write tracks are overlapped, leaving narrow tracks that are fine for the read head, but which can’t be overwritten without destroying the data on adjacent tracks.
In practice, the tracks on an SMR disk are laid down in bands of tracks. The band enables a partial rewrite of the disk by way of an expensive read-modify-write cycle like that used in RAID 5 arrays.
Facebook is using – perhaps testing is more a more accurate term – WD SMR drives in its new 65,000 square foot cold storage data center in Prineville, Oregon. FB’s spec – Cold Storage Hardware – reveals that SMR drives are very sensitive to vibration – only 2 run concurrently in a 30 drive shelf – which is probably why there’s no drive vendors promoting them to consumer or enterprise prospects.
Should the rewrite process intelligence be in the drive or on the host? WD is taking the “dumb drive” approach – let Facebook’s engineers worry about it – rather than handling rewrites within the drive, which I expect is the Seagate approach. This may explain why Seagate will ship a 5TB SMR drive next year, while WD promises 6TB: more and smaller track bands are easier to handle in-drive, but reduce density.
The StorageMojo take
I’ve been agitating for specialized archive disks for years. Low duty cycle drives need to be engineered differently – for example, lubricants – and SMR density helps.
The vibration problem is serious, because vibration has been a disk problem for decades and is only managed, not solved. It may not be possible to design a reliable consumer SMR system at an affordable price. But the enterprise should be a target.
But the amazing thing is that this major development has been going on for at least a year with little in the way of reporting. I’d guess that no more than 200-300 people are involved in this worldwide: a few dozen engineers at WD and Seagate; perhaps a dozen qual engineers each at FB, Amazon, Google, and Microsoft; and another few dozen sales, financial and managerial types. That makes it easy to keep quiet.
Even though one of the same people who developed RAID – Garth Gibson – has been promoting SMR for the last couple of years, the limited number of customers who’d order 10 million of these drives has kept their visibility much lower than RAID had in 1990. The impact of cloud computing goes deep – and SMR drives are another example of how the balance of innovation has shifted from the enterprise to the cloud.
Comments welcome, of course. A version of this post appeared first on ZDNet’s Storage Bits. BTW, SMR seems a natural for object storage systems. What do you think?