Exploring WD's Advanced Format HD Technology

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Western Digital recently began to ship a new series of Caviar hard drives that included the company's Advanced Format technology. The new Caviar Green models are nearly identical to their standard brethren, but offer double the cache (64MB instead of 32MB at 1-2TB) and have a different model number. A WD10EARS is an Advanced Format drive; a WD10-EADS is a 'normal' drive. WD isn't marketing Advanced Format much at the moment, but it's important to understand what the technology is and how it works, particularly if you're still running Windows XP.

Understanding Advanced Format


The new data label for Advanced Format drives. Pay attention.


Hard drive sizes are typically given in terms of total storage capacity, where 1 byte = 10 bits. This is sometimes further broken down by the number of platters and the size of each. The first 1TB drives, for example, used five 200GB platters; current-generation 1TB drives use two 500GB platters. These values, however, only refer to the accessible storage capacity, not the total size of the platter itself. This invisible (to the end-user) additional capacity is used to store positional information and for error correction code.

Advanced Format changes a hard drive's sector size from 512 bytes (the standard for the past three decades) to 4K. This allows the ECC data we referred to above to be stored more efficiently. When a 512 byte sector size is used, Sync/DAM and ECC information is stored as follows:


Old and busted...

Each one of those ECC blocks is 40 bits wide; a 4K block of data contains 320 bytes of ECC. Using Advanced Format's new 4096 sector size cuts the amount of ECC and Sync/DAM space significantly. According to WD, it needs just 100 bytes of ECC data per 4096 byte sector under the new scheme, a savings of 220 bytes.


New hotness.

Debunking the Myth of Additional Drive Space
We want to clear up some confusion regarding the near-term benefits of Advanced Format. In Western Digital's whitepaper on the
subject (PDF), the company states that it can "gain approximately 7-11% in disk space" by using Advanced Format. ECC accounts for 5.5 percent of this; the rest is presumably a mix of efficiency gains in other areas. This has been misinterpreted in a number of circles as meaning that an Advanced Format HDD offers more storage capacity than a normal one. It doesn't—or at least, it doesn't yet. A WD10EARS and a WD10EADS have exactly the same unformatted capacity and Windows reports both drives offer 931GB of storage space.

Western Digital isn't lying about the efficiency benefits of a 4K sector drive, but the company can use that space in a number of ways. Smaller platters are one option, larger storage capacity is another, and removing the innermost tracks of the platter is a third. This last contains an extra bonus—because read and write speeds are typically reported as an average, knocking off the slowest tracks would make the hard drive look faster in a benchmark without actually changing performance at all. For now, WD isn't claiming that Advanced Format delivers any particular advantage and AF drives aren't carrying much of a premium, if any.

The Windows XP Problem


Advanced Format drives emulate a 512 byte sector size, to keep backwards compatibility intact, by mapping eight logical 512 byte sectors to a single physical sector. Unfortunately, this creates a problem for Windows XP users. By default, Windows XP creates a primary disk partition at LBA (logical block address) 63, which is one block short of being evenly divisible by eight. As a result of this offset, data is written across both sides of the 4K physical sector boundary. Read speeds and sequential write speeds aren't as badly impacted by the offset, but the impact on small and random writes is murderous.

The good news is, Western Digital has already solved the problem. Those of you who want to use an AF drive in Windows XP can either install a hardware jumper (if you plan to use a single, simple partition) or run a software tool called
WDAlign. Either solution will restore the drive's full write performance, but WDAlign is what you'll need to use if you've created multiple partitions on a single disk. For our test, we compared the performance of a Caviar Black 1TB (32MB cache, 7200 RPM), and an Advanced Format Caviar Green 1TB (64MB cache, 5400RPM) in 32-bit Windows XP. The Caviar Green was tested both properly aligned and unaligned to highlight the impact of not using WDAlign or setting the requisite jumper.


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