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Wednesday, May 19, 2010
3TB Hard Disk Drives And Their Issues With Legacy OS


Seagate's first 3TB hard disk drive is expecetd to be released later this year. However, not all the operating systems will be able to fully take advantage of the disk' s huge capacity, unless Seagate come up with a solution.

There are two large issues at hand in order to make the new 3TB hard disks actually work: 4K Sector readiness, and the looming 2.1TB limitation that goes along with 512K addressing.

The 4K Sector issue

There is a pending change in the hard drive industry commonly referred to as 4K sectors. The entire hard drive industry is expected to 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. 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.

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.



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 robust 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.

So what's the story? As Seagate's Dave Burks explains, "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. Burks said that there are also fixes in place for Linux.

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

It is obvious that Windows XP will not be able to enjoy the new high-capacity HDDs, at least at their full capacity.

System builders, integrators and IT organizations frequently rely on cloning and imaging software 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, " Burks says.

In the near term, users of 4K drives should pay attention to how their hard drive partition is created. Seagate suggests users to contact their vendor to see if they have an update for their utility software. Users should also find out if their 4K formatted hard drive has other solutions, such as a format utility to re-align their hard drive partitions.

Seagate has not started to ship 4K formatted drives into the general distribution market. However, the company plans to eventually offer 4K drives into the market and promises to offer a a solution that will make all of these alignment issues moot and of no concern to all users.

Seagate's new 3TB drive is expected to be part of Seagate's Constellation ES lineup and will feature a 6Gbit/s SAS interface, along with a spin rate of 7200 rpm.

The 2.1TB limitation

The other issue to be seen is the 2.1TB limitation. Some of the basic design decisions made in the original computer architecture left the industry with inherent limitations. One of those limitations is the ability to address hard drives that exceed capacities of 2.1TB. The decision was made back in 1980 to limit the LBA (logical block address) range to 2.1TB - more than enough capacity in those early days. However, hard disk industry is now faced with operating systems, BIOS controllers, HDD controllers and device drivers that use the same basic limitation of 2.1TB for the maximum size of a hard drive or logical storage device. Long LBA addressing (LLBA) is needed. LLBA extends the number of bytes used in a Command Descriptor Block (a data structure used to format data passed between host computers and hard drives) to allow access to an LBA range that exceeds the 2.1TB limitation.

As a result, in order to enjoy high capacities in the near future, software and hardware suppliers should implement on Long LBAs.


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