Are you ready for 4k sector drives?

The industry is shipping new 4k drives that drop the ancient 512 byte disk sector for a 4096 byte - 4k - sector. What's in it for you? And what will it do to you? Here's what you need to know.
Written by Robin Harris, Contributor

WD has started shipping drives that drop the ancient 512 byte disk sector for a 4096 byte - 4k - sector. What's in it for you? And what will it do to you?

Disks write your data in fixed chunks. For several decades those chunks have been almost always been 512 byte sectors (some vendors have played with other sizes - such as 520 bytes - and irritated their customers no end). But the industry is beginning to phase in a 4k block. Here's what you need to know.

Today's sectors look like this:

courtesy Western Digital

courtesy Western Digital

Why? 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 make it easier to fix those gaps.

The raw error rate is cleaned up by the sector error correcting code (ECC) and sophisticated signal processing to reach a SATA drive's specified error rate of 1 in 1014. The magnetic spots are smaller than 45nm transistors and they're spinning 120 times a second. Without robust ECC disk storage wouldn't work.

Why now? A 512 byte sector can't support enough ECC to correct for increasing 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.

Here's a diagram of the 4k sector. Less protocol overhead and better ECC.

courtesy Western Digital

courtesy Western Digital

Note: the longer ECC doesn't change the drive level read error rate. It remains at 1 unrecoverable read error about every 12.5 TB.

Is this new? 4k sectors have been cooking for over a decade. Drive vendors started by convincing Microsoft and other OS and BIOS vendors of the problem years ago.

The late adopters are the cloning software vendors. More on that in a moment.

Why should I care? Would you like a 4 TB disk? 8 TB?

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.

What about performance? Marginal, invisible-to-the-naked-eye improvements. But it won't be worse, either.

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?

The emulation mode puts 8 512 byte writes into a single 4k sector. There is no loss at all. Here's a picture:

courtesy Western Digital

courtesy Western Digital

Gotchas? 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. WD-branded 4k USB drives are already aligned so it isn't needed for those 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.

How can I recognize a 4k drive? WD is labeling theirs and I assume other vendors will follow suit. Here's WD's label:

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

XP users have a bit more to be aware of and some will get bit by performance issues. The easiest solution for XP users: avoid installing 4k drives. Factory installed XP will be fine.

My question: why not a better read-error spec? Today's large SATA drives shouldn't be used in 4 drive 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.

Comments welcome, of course. WD's dynamic Heather Skinner arranged my briefing. No sectors, old or new, changed hands.

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