Dell PowerEdge R530 - Manual (Page 5)

5 512e and 4Kn Disk Formats
2 Background
The legacy sector format contains a Gap section, a Sync section, an Address Mark section, a Data section
and Error Correction Code (ECC) section as shown in Figure 1.
Figure 1 Legacy sector format
The sector layout is structured as follows:
• Gap"section:"The"gap"separates"sectors.
• Sync"section:"The"sync"mark"indicates"the"beginning"of"the"sector"and"provides"timing"alignment.
• Address"Mark"section:"The"address"mark"contains"data"to"identify"the"sector’s"number"and"location."
It"also"provides"status"about"the"sector"itself.
• Data"section:"The"data"section"contains"all"of"the"user’s"data.
• ECC"section:"The"ECC"section"contains"error"correction"codes"that"are"used"to"repair"and"recover"
data"that"might"be"damaged"during"the"reading"or"writing"process.
This low-level format has served the industry well for many years. However, as hard drive capacities have
increased, sector size has increasingly become a limiting design element in improving hard drive capacities
and error correction efficiency. For example, comparing the ratio of sector size to the total capacity of
earlier hard drives to more recent drives, the sector resolution has become extremely small. The resolution
of the sector (the ratio of sectors as a percentage of total storage) has become very fine and increasingly
inefficient as shown in Table 2.
Table 2 Sector resolution
Capacity Total sectors Sector resolution
40MB 80,000 .001%
400GB 800,000,000 .0000001%
Very fine resolution is good when managing small, discrete amounts of data. However, applications
common in modern computing systems manage data in large blocks, much larger in fact than the legacy
512-byte sector size. More importantly, the small 512-byte sector has consumed a smaller and smaller
amount of space on the hard drive surface as areal densities have increased.
The migration to larger sectors within the hard drive industry has become a fundamental need relative to
gaining improvements in error correction and achieving format efficiencies.