Is there a shingled disk in your future?

Disk vendors are running out of options when it comes to increasing disk drive density. One increasingly attractive option: Shingled Magnetic Recording (SMR). What is it and will it succeed?

What is shingled magnetic recording?

In today's disk drives tracks are separated by a small space or gap. In SMR drives, this gap is removed to achieve greater density.

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 cannot be easily overwritten without destroying the data on one or more adjacent tracks.

In practice, the tracks on an SMR disk would be laid down in groups or bands of tracks. The band enables a partial rewrite of the disk. How big that rewrite is depends on the size of the band.

Why is SMR attractive?

Seagate is researching SMR because the major alternatives, Heat-Assisted Magnetic Recording (HAMR) and Bit-patterned Media (BPM) look to require massive investments that drive vendors - hit by the drop-off in PC sales - may not be able to afford (for more on HAMR and BPM, see Engineering the 10TB notebook drive). SMR drives can be built using today's production lines and technology - with 1 significant exception.

The SMR question mark

The downside of SMR is the fact that you can't rewrite data in place: you have to rewrite an entire band of tracks. However, this is similar to the problem that NAND flash controller developers have been grappling with the last 5 years: you have to rewrite an entire 256k block (or greater) of flash, not just a 4k page. 

As data is deleted and capacity freed up, the remaining good data has to be collected and rewritten so unused capacity can be reclaimed, a process known at garbage collection. Managing that garbage collection and other processes requires the same kind of virtualization of the disk blocks flash controllers have been performing on SSDs.

But another option is changing the file system to treat an SMR disk more like a tape device: gathering blocks to be written together and then writing all of them at the same time. Seagate has supported academic research into this option at Carnegie-Mellon University.

The Storage Bits take

Drive vendors Seagate, WD and Toshiba have been hit with a double-whammy: SSDs have devastated their high-margin enterprise drive business; and tablets and smartphones are stunting their bread-and-butter PC drive business. They've finally moved into the higher-margin add-on storage business, but I expect anemic financial results for the next several years. 

If they don't preserve their $/GB cost advantage over SSDs they'll lose even more volume. SMR looks to be the easiest way to keep growing areal density, but they'll need much more sophisticated drive controllers or file systems to make it work. 

The controller model will be the fastest to implement, but at higher product cost. New file systems would mean lower costs for drives, but they are years away, if ever. 

The disk drive business has never been easy. It's looking even tougher going forward.

Comments welcome, of course. Hybrid drives may offer the easiest path to SMR, since there is a controller and plenty of flash already on the drive. But do you want a 10TB notebook drive?


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