Saturday, February 04, 2012

Multiple arms/actuators and shingled-write drives

Previously, I've written on last-gen (Z-gen) shingled-write drives and mentioned multiple independent arms/actuators:
  • separating "heavy" heads (write + heater) from lightweight read-only, and
  • using dual sets of heads, either read-only or read-write.
There is some definitive work by Dr. Sudhanva Gurumurthi of University of Virginia and his students on using multiple arms/actuators in current drives, especially those with Variable Angular Velocity, not Constant Angular Velocity, maybe nearer Constant Linear Velocity - approaches used in Optical drives.
E.g. "Intra-disk Parallelism" thesis and "Energy-Efficient Storage Systems" page.

The physics are good and calculations impressive, but where is the commercial take-up?
Extra arms/actuators and drive/head electronics are expensive, plus need mounting area on the case.
What's the "value proposition" for the customer?

Both manufacturers and consumers have to be convinced it's a worthwhile idea and there is some real value. Possibly the problem is two-fold:
  • extra heads don't increase capacity, only reduce seek time (needed for "green" drives), a hard sell.
  • would customers prefer two sets of heads mounted in two drives with double the capacity, the flexibility to mirror data and replace individual units.
Adopting dual-heads in shingled-write drives might be attractive:
  • shingled-write holds the potential to double or more the track density with the same technology/parts. [Similar to the 50% increase early RLL controllers gave over MFM drives.]
    Improving drive $$/GB is at least an incentive to produce and buy them.
  • We've no idea how sensitive to vibration drives with these very small bit-cells will be.
    Having symmetric head movement will cancel most vibration harmonics, helping settling inside the drive and reducing impact externally.
To appreciate the need

Ed Grochowski, in 2011 compared DRAM, Flash and HDD,  calculating bitcell sizes for a 3.5 in disk and 750Gb platter used in 3TB drives (max 5 platters in a 25.4 mm thick drive).

The head lithography is 37nm, tracks are 74nm wide and now with perpendicular recording, 13nm long.
The outside track is 87.5mm diameter, or 275 mm in length, hold a potential 21M bitcells, yielding 2-2.5MB of usable data. With 2KB sectors, ~1,000 sectors/track maximum.

The inner track is 25.5mm diameter, 80mm in length: 29% of the outside track length.
The 31mm wide write-area contains up to 418,000 tracks with a total length of ~5 km.
Modern drives group tracks in "zones" and vary rotational velocity. The number of zones and how closely  they approximate "Constant Linear Velocity", like early CD drives, isn't discussed in vendors data sheets.

While Grocowski doesn't mention clocking or sector overheads (headers, sync bits, CRC/ECC) and inter-sector gaps.
Working backwards from the total track length, the track 'pitch' is around 150nm, leaving a gap of roughly a full track width between tracks.

I've not seen mentioned bearing runout and wobble that require heads to constantly adjust tracking, a major issue with Optical disks. Control of peripheral disk dimensions and ensuing problems is discussed.

As tracks become thinner, seeking to, and staying on, a given track becomes increasingly difficult. These are extremely small targets to find on a disk and tracking requires very fine control needing both very precise electronics and high-precision mechanical components in the arms and actuators.

Dr Gurumurthi notes in "HDD basics" that this "settling-time" becomes more important with smaller disks and higher track density.

This, as well as thinner tracks, is the space that shingled-writing is looking to exploit.
The track width and pitch become the same, around 35nm for a 4-fold increase in track density using current heads, less inter-region gaps and other overheads.

Introducing counter-balancing dual heads/actuators may be necessary to successfully track the very small features of shingled-write disks. A 2-4 times capacity gain would justify the extra cost/complexity for manufacturers and customers.

No comments: