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In-Depth Information
and more bits, requiring more ECC bytes and more complex calculations just to
maintain
a given error rate. A 12-byte ECC was enough to achieve a 1 in 1014 error rate back in
1990; however, this grew to 24 bytes by 1999 and 40 bytes by 2004 just to maintain the
same error rate. In other words, even without decreasing the error rate further, the num-
ber of ECC bytes has grown from 12 bytes to 50 bytes, creating a significant amount of
storage overhead. By 2010, it was found that additional increases in ECC capability for
512-byte sectors would require the number of ECC bytes to increase to the point that it
wouldnegateanyfurtherincreasesindensity.Inotherwords,astalemateofsortshasbeen
reached between density and ECC capability.
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
totalnumberofgaps,headers,andtrailersoverall.ThisallowsforalargerECCtobeused,
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.Themoveto4Ksectorswasmadenecessary
due to density increases demanding more powerful ECC (error correction code) calcula-
tions, 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 Asso-
ciation (IDEMA,
www.idema.org
) formed the 4K Block Committee to study and pro-
mote the transition to 4K sectors. IDEMA is a trade organization founded in 1986 and
consisting of drive manufacturers, component suppliers, and even software compan-
ies—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 FreeA-
gent 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
beAdvancedFormat(4Ksector)drives.
Table9.6
showstheformatofa4,096-bytesector
on a modern 4K sector (Advanced Format) drive.
Table 9.6 Typical 4K-Byte Sector Format
