Hardware Reference
In-Depth Information
Fortunately, you can realize a much greater improvement in ECC and sector efficiency by going to
larger sectors—that is, increasing the size of the data area, thereby reducing the total number of gaps,
headers, and trailers overall. This allows for a larger ECC to be used, while still resulting in greater
sector efficiency overall.
Advanced Format (4K Sectors)
When the first PC was introduced in 1981 and for the next 28 years that followed, all the disk drives
used in PCs stored exactly 512 bytes of data in each physical sector. This de facto standard began to
change in 2009 when the first hard drives storing data in 4,096-byte physical sectors were released.
Drives that store 4,096 bytes per sector are called
4K sector
or
Advanced Format
drives. The move
to 4K sectors was made necessary due to density increases demanding more powerful ECC (error
correction code) calculations, which were taking up an increasing amount of space in each sector.
The transition to 4K sector drives can be traced as far back as 1998 when IBM released a paper
identifying the 512-byte sector format as a bottleneck to increased drive capacity and performance. In
2000, the International Disk Drive Equipment and Materials Association (IDEMA,
www.idema.org
)
formed the 4K Block Committee to study and promote the transition to 4K sectors. IDEMA is a trade
organization founded in 1986 and consisting of drive manufacturers, component suppliers, and even
software companies—basically anybody producing drives or related products. Seagate began
shipping 4K sector drives in May 2009 inside USB-attached external hard drives such as the
FreeAgent series, whereas Western Digital released the first bare 4K sector drive in December 2009.
These products were the first wave in a major change in disk drive designs, as the drive industry as a
whole agreed that all new drive platforms introduced after 2011 would be Advanced Format (4K
sector) drives.
Table 9.6
shows the format of a 4,096-byte sector on a modern 4K sector (Advanced
Format) drive.
Table 9.6. Typical 4K-Byte Sector Format
As you can see, the 4K sector (aka Advanced Format) layout uses the same number of bytes for the
sector header as modern 512-byte sectored drives, but it increases the size of the ECC field to 100
bytes. Even with the larger ECC required to maintain the standard error rate, this yields a sector
format efficiency of 97.3%—almost a 10% improvement. Because of the eight times larger data field,
this will also allow further increases in ECC without dramatically reducing the overall efficiency.
Figure 9.4
shows the format efficiency improvement from 512-byte sectors to 4K sectors.
