Advanced Format “4K Sectors” Explained
Advanced Format “4K Sectors” Explained
There is one feature included on the Seagate XT 3TB we need to take a look at: “Advanced Format” or 4K sector formatting. This feature isn’t exactly new since Seagate implemented it with their 2TB hard drive a while ago, but it is and will be an integral part of higher capacity drives. The reasoning for this vaires, but it basically boils down to the fact that in order to move to larger and larger hard drives, 512 byte sector formatting had to be phased out and replaced with 4k sectors due to Master Boot Record limitations. If they had not done so, your fancy new 3TB drive would be at best be turned into a 900 GB drive since the system would ignore everything below the 2.1TB mark and “see” only the portion above it. At worst, it wouldn’t be recognized at all.
OS limitations are the main reason for going to 4K sectors, but there is a secondary side effect to this feature as well. Basically, using 4K sectors instead of the much older 512 byte sectors involves error correction or “Error Correcting Code” (ECC for short).
ECC is used overcome the inherent increase in “noise” that is introduced when making platters with higher densities. No matter how precise write and read heads are, the closer the bits are together the higher chances that there will be bleed over from one to another. In a nutshell, the bits are now so close together that when a drive goes to write in a given bit there is a small chance that the bit next to it will also be negatively impacted. Much like on a CD, ECC is literally additional bits added on to a sector that tells the controller what the data should be and how to correct it if there are deviations.
By going to 4K sectors, not only can manufactures use ECC that is more effective (estimated at upwards of 200% more effective) but it also take up less space. The “claim” of taking up less space and yet being more effective may at first seem counter intuitive since the data should take up just as much room in 4K sectors as it does in 512 byte sectors and be just as effective.
In the old 512 byte sector layout, each little sector had 40 bytes of ECC clustered which was responsible for that particular sector. On the surface, one would think you would need 320 bytes of ECC for the new larger 4K sector but since the implementation of 512 byte sectors, the algorithms behind ECC have become more refined and compact. By going with more advanced algorithms, the new 4K sector layout still requires one cluster of ECC at the end of a sector, but that cluster is now only about 100 bytes in size.
As an added bonus, 4K sectors are not only more compact but also quicker and easier to create and read. Since the hard drive controller only needs to make one ECC per sector but that sector is now 4 times as large, the overall ECC creation is faster. It takes processor cycles to create ECC, so the less time the controller has to work on ECC the more time it has for doing other things.
Years ago when storage size was actually at a premium the less “waste” you had between files, the more room you could use. This additional wasted space was called “slack space” and was essentially useless as it couldn’t be used. Moving to 4K sectors obviously means more slack space, but with monster-capacity drives it no longer matters since physical storage space has been gradually reduced in overall cost.
Sadly, not everything is wine and roses with 4K sectors and the additional wasted space is not the most significant downside either. The main issue is the potential performance impact of 4K sectors when used in conjunction with older operating systems.
As any SSD user will tell you, XP doesn’t use a “proper” offset when it formats a drive since it uses a less than optimal 63-bit offset. This was perfectly fine in the past but on any storage device that uses 4K sectors, this offset will cause issues since a single 4K write will use two sectors instead of one.
To get around this issue Seagate has taken a slight different approach than Western Digital. Unlike Western Digital that has specialty software plus a hardware option, Seagate has opted to simply update their tried and true Seagate DiscWizard.
The upgraded version of this venerable setup and installation program will allow older XP computer users the ability to partition their 3TB beast into two (or more) partitions. It will also install a driver allowing the XT to be a bootable drive. Of course, you will not be able to use all 3TB in one partition on 32-bit operating systems, but it certainly is impressive and goes a long ways towards making up for the fact that Seagate does not include a HBA adapter card like Western Digital does nor a 4K offset jumper option on the drive itself.
