Disk sector
In computer
The sector is the minimum storage unit of a hard drive.[1] Most disk partitioning schemes are designed to have files occupy an integral number of sectors regardless of the file's actual size. Files that do not fill a whole sector will have the remainder of their last sector filled with zeroes. In practice, operating systems typically operate on blocks of data, which may span multiple sectors.[2]
Geometrically, the word
In modern disk drives, each physical sector is made up of two basic parts, the sector
History
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
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
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][failed verification]
In the 1970s, IBM added
In 2000 the industry trade organization, International Disk Drive Equipment and Materials Association (
Related units
Sectors versus blocks
While sector specifically means the physical disk area, the term block has been used loosely to refer to a small chunk of data. Block has multiple meanings depending on the context. In the context of data storage, a filesystem block is an abstraction over disk sectors possibly encompassing multiple sectors. In other contexts, it may be a unit of a data stream or a unit of operation for a utility.[12] For example, the Unix program dd allows one to set the block size to be used during execution with the parameter bs=bytes
. This specifies the size of the chunks of data as delivered by dd, and is unrelated to sectors or filesystem blocks.
In Linux, disk sector size can be determined with sudo fdisk -l | grep "Sector size"
and block size can be determined with sudo blockdev --getbsz /dev/sda
.[13]
Sectors versus clusters
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In computer file systems, a cluster (sometimes also called allocation unit or block) is a unit of disk space allocation for files and directories. To reduce the overhead of managing on-disk data structures, the filesystem does not allocate individual disk sectors by default, but contiguous groups of sectors, called clusters.
On a disk that uses 512-byte sectors, a 512-byte cluster contains one sector, whereas a 4-
A cluster is the smallest logical amount of disk space that can be allocated to hold a file. Storing small files on a filesystem with large clusters will therefore waste disk space; such wasted disk space is called
A cluster need not be physically contiguous on the disk; it may span more than one
A "lost cluster" occurs when a file is deleted from the directory listing, but the File Allocation Table (FAT) still shows the clusters allocated to the file.[14]
The term cluster was changed to allocation unit in DOS 4.0. However the term cluster is still widely used.[15]
Zone bit recording
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
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.
Advanced Format
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
See also
- CD-ROM format
- Count key data
- Cylinder-head-sector
- Disk formatting
- Disk storage
- File Allocation Table (FAT)
- Hard disk drive partitioning
- Sector slipping
References
- ISBN 9788189093242.
- ISBN 9780203494455.
- ^ 305 RAMAC Random Access Method of Accounting and Control Manual of Operation (PDF). IBM. 1957.
- ^ IBM 1301, Models 1 and 2, Disk Storage and IBM 1302, Models 1 and 2, Disk Storage with IBM 7090, 7094, and 7094 II Data Processing Systems (PDF). IBM. A22-6785.
- ^ IBM 1301, Models 1 and 2, Disk Storage and IBM 1302, Models 1 and 2, Disk Storage with IBM 1410 and 7010 Data Processing Systems (PDF). IBM. A22-6788.
- ^ Technical Data - Series 4000 Disk File (PDF). Bryant Computer Products. 1963.
- ^ Porter, James (October 1988). "Rigid Magnetic Disk Drive Specifications". 1988 DISK/TREND REPORT, RIGID DISK DRIVES. DISK/TREND, Inc. p. 63, 122.
- ^ Porter, James (June 1997). "Rigid Magnetic Disk Drive Specifications". 1997 DISK/TREND REPORT, RIGID DISK DRIVES. DISK/TREND, Inc.
- ^ Reference Manual for IBM 3330 Series Disk Storage (PDF). IBM. March 1974. GA26-1615-3.
- ^ a b c "The Advent of Advanced Format". IDEMA. Retrieved 2013-11-18.
- www.idema.org. Archivedfrom the original on 14 December 2020. Retrieved 14 December 2020.
- ^ "Difference between block size and cluster size". unix.stackexchange.com. Retrieved 2015-12-13.
- ^ "Disk Sector and Block Allocation For File". stackoverflow.com. Retrieved 2015-12-13.
- ^ "Errors Caused by Cross-Linked Files or Lost Clusters". Archived from the original on 2015-03-06. Retrieved 2020-08-03.
- ISBN 0-7897-2745-5.
- ^ Kern Wong (January 1989), DP8459 Zoned Bit Recording (PDF), National Semiconductor, archived from the original (PDF) on 2011-06-15, retrieved 2010-03-10