WD has started shipping drives that drop the ancient 512 byte disk sector for a 4096 byte – 4k – sector, and the rest of industry isn’t far behind. For several decades disk sectors have been almost always been 512 bytes (NetApp tried 520 bytes – and irritated their customers no end). Why 4k and why now?

Rising bit density means smaller magnetic areas and more noise. The underlying or raw disk media error rate is approaching 1 error in every thousand bits on average – while tiny media defects can lose hundreds of bytes in a row. The larger sectors enable more powerful ECC to fix those gaps.

Why now?
A 512 byte sector can’t support enough ECC to correct for higher raw error rates. Thus bigger sectors with stronger ECC capable of detecting and correcting much larger errors – up to 400 bytes on a 4k sector.

The 4k sector enables disk manufacturers to keep cramming more bits on a disk. Without them the annual 40% capacity increases we’ve come to expect would stop.

Note: the longer ECC doesn’t change the drive level unrecoverable read error rate. It remains at 1 in every 1014 bytes.

4k sectors have been cooking for over a decade. The late adopters are the cloning software vendors. More on that in a moment.

Will 4k sectors use capacity faster?
If you write 500 bytes and the minimum sector is 4k, will that write take up the full 4k, wasting 3.5 KB? No.

The initial WD drives – and I assume other vendors as well – will operate in a 512 byte emulation mode. Eventually new disks will operate in native 4k mode, and then you might have a concern. But many operating systems already do 4k IO. And at a couple of cents per future GB, who cares?

If you are in either of these 2 groups:

  1. Windows XP users
  2. Windows users who clone disks with software like Norton Ghost

there are a couple of gotchas if you want to use a 4k drive. Since most drives aren’t 4k and won’t be for another year or more, this may not affect you either. Vista and W7 users are cool except for cloning.

1) Windows XP does not automatically align writes on 4k boundaries, which hurts performance. WD has software – the Advanced Format Align Utility for their drives. I assume other vendors will too when they start shipping.

XP users need to run this utility once to use a 4k drive with a clean install, cloning software or a do-it-yourself USB drive. It isn’t needed for WD-branded 4k USB drives.

2) Windows clone software vendors have yet to implement 4k support. If you clone an XP, Vista or W7 drive you should run the align utility. The cloning vendors need to get on board Real Soon Now. Vendors are welcome to comment on their plans.

What about Macs?
No worries: Mac OS just works with 4k drives – including cloning.

There’s been a lot of heavy lifting behind the scenes to make this a smooth transition. With Vista, W7, Mac OS and Linux support well in hand most users won’t notice any change.

Some XP users will get bit by performance issues. The easiest solution for XP users: avoid 4k drives. Factory installed XP will be fine.

The StorageMojo take
My question: why not a better read-error spec? Today’s large SATA drives shouldn’t be used in RAID 5 arrays due to the high likelihood of a read error after a drive failure, which will abort the RAID rebuild. A better error spec would fix this.

Oh, RAID 6 sells more drives? Never mind.

Finally, the drive industry doesn’t know how to talk to consumers about technology. It took me an hour of digging to understand how this benefits consumers rather than vendors.

Comments welcome, of course. WD’s dynamic Heather Skinner arranged a briefing for me. No sectors, old or new, changed hands.