Disk sector

The first disk drive, the 1957 IBM 350 disk storage, had ten 100 character sectors per track; each character was six bits and included a parity bit. The number of sectors per track was identical on all recording surfaces. There was no recorded identifier field (ID) associated with each sector.[3]

The 1961 IBM 1301 disk storage introduced variable length sectors, termed records or physical records by IBM, and added to each record a record address field separate from the data in a record.[4][5] All modern disk drives have sector address fields, called ID fields, separate from the data in a sector.

Also in 1961 Bryant with its 4000 series introduced the concept of zoned recording (ZBR) which allowed the number of sectors per track to vary as a function of the track's diameter – there are more sectors on an outer track than on an inner track.[6] In the late 1980s ZBR was again used in disk drives then announced by Imprimis and Quantum[7] and by 1997 its industry usage was ubiquitous.[8]

The disk drives and other DASDs announced with the IBM System/360 in 1964 used self-formatting variable length sectors, termed records or physical records by IBM. They detected errors in all fields of their records with a cyclic redundancy check (CRC) replacing parity per character detection of prior generations. These IBM physical records have three basic parts, a Count field which acts as an ID field, an optional Key field to aid in searching for data and a Data field; in practice, most records had no Key field, indicated by a key length of zero. The structure of these three fields is called the CKD track format for a record.

The 1970 IBM 3330 disk storage replaced the CRC on the data field of each record with an error correcting code (ECC) to improve data integrity by detecting most errors and allowing correction of many errors.[9] Ultimately all fields of disk sectors had ECCs.

Prior to the 1980s, there was little standardization of sector sizes; disk drives had a maximum number of bits per track and various system manufacturers subdivided the track into different sector sizes to suit their OSes and applications. The popularity of the PC beginning in the 1980s and the advent of the IDE interface in the late 1980s led to a 512-byte sector becoming an industry standard sector size for HDDs and similar storage devices.[10]

In the 1970s, IBM added fixed-block architecture Direct Access Storage Devices (FBA DASDs) to its line of CKD DASD. CKD DASD supported multiple variable length sectors while the IBM FBA DASD supported sector sizes of 512, 1024, 2048, or 4096 bytes.

In 2000 the industry trade organization, International Disk Drive Equipment and Materials Association (IDEMA) started work to define the implementation and standards that would govern sector size formats exceeding 512 bytes to accommodate future increases in data storage capacities.[10] By the end of 2007 in anticipation of a future IDEMA standard, Samsung and Toshiba began shipments of 1.8-inch hard disk drives with 4096 byte sectors. In 2010 IDEMA completed the Advanced Format standard for 4096 sector drives,[10] setting the date for the transition from 512 to 4096 byte sectors as January 2011 for all manufacturers,[11] and Advanced Format drives soon became prevalent.

If a sector is defined as the intersection between a radius and a track, as was the case with early hard drives and most floppy disks, the sectors towards the outside of the disk are physically longer than those nearer the spindle. Because each sector still contains the same number of bytes, the outer sectors have lower bit density than the inner ones, which is an inefficient use of the magnetic surface. The solution is zone bit recording, wherein the disk is divided into zones, each encompassing a small number of contiguous tracks. Each zone is then divided into sectors such that each sector has a similar physical size. Because outer zones have a greater circumference than inner zones, they are allocated more sectors. This is known as zoned bit recording.[16]

A consequence of zone bit recording is that contiguous reads and writes are noticeably faster on outer tracks (corresponding to lower block addresses) than on inner tracks, as more bits pass under the head with each rotation; this difference can be 25% or more.

In 1998 the traditional 512-byte sector size was identified as one impediment to increasing capacity which at that time was growing at a rate exceeding Moore's Law. Increasing the length of the data field through the implementation of Advanced Format using 4096-byte sectors removed this impediment; it increased the efficiency of the data surface area by five to thirteen percent while increasing the strength of the ECC which in turn allowed higher capacity. The format was standardized by an industry consortium in 2005 and by 2011 incorporated in all new products of all hard drive manufacturers.