Competition heats up SMR drives

Competition heats up SMR drives

Summary: Despite slowing areal density increases drive makers have other tricks to increase capacity. Next up, SMR drives that will double disk capacity - at a price.

TOPICS: Storage

WD has been sampling SMR - Shingled Magnetic Recording - disk drives to several cloud vendors including Facebook, Google, Microsoft and Amazon. In September Seagate announced sales of a million SMR drives - I didn't know they'd sold any - and is planning a 5TB SMR drive for next year. HGST - an independent unit of WD - likely has a similar program under way.

What is SMR?
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.

Cold storage
Facebook is testing Western Digital SMR drives in its new 65,000 square foot cold storage data center in Prineville, Oregon. FB found that 8% of photos got 82% of the accesses, which means over 90% of the photos share the remaining 18% of accesses.

FB knew that few cold photos would ever be accessed, but they still wanted them available just in case - which ruled out tape. Storing them on SMR disks maximizes capacity at little extra cost.

FB has published a spec - Cold Storage Hardware (pdf) - that illuminates the limits of SMR drives. The big issue: SMR drives are very sensitive to vibration - only 2 can run concurrently in a 30 drive shelf - which is probably why there's no marketing aimed at consumer or enterprise prospects.

Another issue is managing the rewrite process. Should the intelligence be in the drive or on the host? WD is taking the "dumb drive" approach - let Facebook's many software engineers worry about it - rather than handling rewrites within the drive, which I expect is the Seagate approach.

The cloud vendors probably favor the dumb drive approach because then the band size can be larger - using server resources - that a disk controller could handle, giving them more useful capacity at the cost of some CPU cycles.  But perhaps Seagate has some more tricks up its sleeve.

The Storage Bits take
I've been agitating for specialized archive disks for years and it looks like we're getting close. Low duty cycle drives need to be engineered differently - for example, lubricants - and SMR density is helpful too.

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 advent of a reliable archive DVD - and the promise of a reliable archive Blu-ray - means that perhaps consumers won't want tricky 12TB archive disks. Yet our digital civilization needs reliable digital archives - and competing SMR drives from WD and Seagate are a step in the right direction.

Comments welcome, of course. Good to see that drive competition isn't dead.

Topic: Storage

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  • Reminiscent of video tape recording

    In the early days of video tape, there had to be a gap between adjacent (diagonal) tracks as data on adjacent tracks would otherwise get picked up by either of the two heads on opposite sides of the scanning drum and pollute the clean data from the main track. The solution was to alter the azimuth (twist the heads slightly) so that one head couldn't read what the other had laid down. This enabled the gap between tracks to be eliminated and enabled the birth of the domestic video industry as it nearly doubled the capacity of tape.
    I wonder whether something similar could be done with discs. I know disc tracks are very much narrower but so is the data density. I don't like the idea of increasing data density at the risk of losing it due to vibration - that strikes me as a seriously retrograde step.
    I also wonder whether lasers could be used to read and write the data as was done with audio minidiscs a few years ago - could recording densities be increased this way? Using lasers would also eliminate the possibilty of a disc crash as the laser wouldn't need to be near enough to the disc to touch it, unlike flying magnetic heads. So if a disc failed, it would be due to one of 4 things: drive motor failure, laser positioning motor failure, laser failure or electronics failure. In all cases, the data on the disc would be undamaged and could be recovered by transplanting the actual disc(s) into a fully functioning housing.
  • Excellent point about video.

    The resulting technology - embodied in the Sony 8mm drive and OEM'd by Exabyte for data use - transformed the backup tape market.

    But though the density increases are analogous, the tape vs disk issues are very different. Rewriting part of a tape wasn't a big deal; with SMR it is.

    As for lasers, I've been looking at some extremely promising developments in metamaterials. Hope to get something out about them in the next month or so.

    R Harris