4K Sector Hard Drive Primer

OK, let me apologize for the length and perhaps the overly “techy” aspect of this post in advance.  Part of my job at Seagate is to help communicate a pending change in the hard drive industry commonly referred to as 4K sectors.  The entire hard drive industry will transition from legacy 512 byte sectors to 4K sectors on many products over the course of the coming year. This change is inevitable, as all HDD manufacturers have agreed that all new desktop and notebook drives introduced after January 2011 will be 4K formatted drives (OEM relationships do not necessarily apply).  These drives have already started to show up in the market bearing the name “Advanced Format” which was selected and approved as an industry term by the IDEMA organization last December.

In general, this is a good thing for the industry because 4K formatted drives are more efficient in terms of capacity and error correction. This is because one 4K sector saves a lot of overhead compared to the leagacy 512 byte standard while simultaneously growing the size of the ECC (error correction codes) block to make it more robust.  Check out this diagram:

Notice how one 4k sector contains the equivalent data of eight 512 byte sectors.  However, it only needs one set of “overhead” segments (the bytes on the front end used for separation addressing, etc.) and one ECC block.  More importantly, the ECC block doubles in size making it more r0bust while also saving space.

Over time, this will help hard drive manufacturers deliver larger capacities with strong data correction capabilities.  In the short term, there are some minor growing pains to avoid. Here’s the deal. It’s not practical to make host computer systems talk in 4K native language – at least for a while. There are just too many places the 512 byte assumption is embedded.  So, while hard drives will transition to 4K sectors physical sectors on the media, they will still “look and talk” like 512 byte formats to host computers. They will actually emulate 512 byte communications to hosts.  This works well as long as the logical 512 byte assumptions from the host computer are aligned with the 4K sectors on the hard drive.

It turns out that when a hard drive partition is created, the starting position can vary.  A 4K drive format is set to work under the assumption that the first 512 byte sector (Logical Block Address = 0) will align perfectly with the first physical 4K sector, like this:

Alignment 0, as shown above, works well for hard drives & 512 byte emulation because they can neatly map eight 512 byte logical blocks into a single sector.  Sometimes hard drive partitions get created so the logical to physical alignment is off, like this:

This is called Alignment 1 and it’s not so good for 4K drives when it comes to emulating 512 byte legacy sectors, especially when writing data.  Essentially, this alignment can often cause a hard drive to manage a write with extra disc rotations, which slows things down.  The results can be sort of dramatic as shown by these test results at HOTHARDWARE.com.

Now for some good news -if you format and create hard drive partitions with Windows Vista (SP 1 or later) or Windows 7, you’re in good shape. These versions of the windows OS create aligned partitions and everything works well. The same can be said about all versions of Apple MacOS.  There are also fixes in place for Linux, but there are plenty of hybrid versions out there that will need attention. For this blog, I’ll stick to Windows.

There are two categories of software that can create un-aligned partitions for Windows environments:

1) Windows XP:  XP creates un-aligned partitions.

2) Cloning and imaging software: Highly likely to create un-aligned partitions

The Windows XP situation seems fairly well understood in the marketplace. What is much less understood is the situation with cloning and imaging software.  System builders, integrators and IT organizations frequently rely on these tools to configure systems for sale or deployment in their organization.  Even if you are using Windows Vista or Windows 7, if your hard drive was partitioned with one of these utilities, you’re likely to end up with an un-aligned partition with the potential for poor performance.

In the near term, users of 4K drives should pay attention to how their hard drive partition is created with respect to the guidance I just mentioned. If you are not sure about your utility software, contact your vendor to see if they have an update. You should also find out if your 4K formatted hard drive has other solutions, such as a format utility to re-align your hard drive partitions.

As for Seagate, we have not started to ship 4K formatted drives into the general distribution market. However, we will eventually offer 4K drives into the market with a solution that will make all of these alignment issues moot and of no concern to our customers.

More on that in the near future….