computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes, and development of both hardware and software. Computing has scientific, ...

(specifically data transmission and data storage), a block, sometimes called a physical record, is a sequence of

byte The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable uni ...

s or

bit The bit is the most basic unit of information in computing and digital communications. The name is a portmanteau of binary digit. The bit represents a logical state with one of two possible values. These values are most commonly represente ...

s, usually containing some whole number of records, having a maximum length; a ''block size''. Data thus

structured Structuring, also known as smurfing in banking jargon, is the practice of executing financial transactions such as making bank deposits in a specific pattern, calculated to avoid triggering financial institutions to file reports required by law ...

are said to be ''blocked''. The process of putting data into blocks is called ''blocking'', while ''deblocking'' is the process of extracting data from blocks. Blocked data is normally stored in a

data buffer In computer science, a data buffer (or just buffer) is a region of a memory used to temporarily store data while it is being moved from one place to another. Typically, the data is stored in a buffer as it is retrieved from an input device (such a ...

, and read or written a whole block at a time. Blocking reduces the overhead and speeds up the handling of the

data stream In connection-oriented communication, a data stream is the transmission of a sequence of digitally encoded coherent signals to convey information. Typically, the transmitted symbols are grouped into a series of packets. Data streaming has b ...

. For some devices, such as magnetic tape and CKD disk devices, blocking reduces the amount of external storage required for the data. Blocking is almost universally employed when storing data to 9-track magnetic tape, NAND flash memory, and rotating media such as floppy disks, hard disks, and

optical disc In computing and optical disc recording technologies, an optical disc (OD) is a flat, usually circular disc that encodes binary data (bits) in the form of pits and lands on a special material, often aluminum, on one of its flat surfaces. ...

s. Most file systems are based on a

block device In Unix-like operating systems, a device file or special file is an interface to a device driver that appears in a file system as if it were an ordinary file. There are also special files in DOS, OS/2, and Windows. These special files allow a ...

, which is a level of abstraction for the hardware responsible for storing and retrieving specified blocks of data, though the block size in file systems may be a multiple of the physical block size. This leads to space inefficiency due to

internal fragmentation In computer storage, fragmentation is a phenomenon in which storage space, main storage or secondary storage, is used inefficiently, reducing capacity or performance and often both. The exact consequences of fragmentation depend on the specific ...

, since file lengths are often not integer multiples of block size, and thus the last block of a file may remain partially empty. This will create

slack space Slack may refer to: Places * Slack, West Yorkshire, a village in Calderdale, England * The Slack, a village in County Durham, England * Slack (river), a river in Pas-de-Calais department, France * Slacks Creek, Queensland, a suburb of Logan City, ...

. Some newer file systems, such as

Btrfs Btrfs (pronounced as "better F S", "butter F S", "b-tree F S", or simply by spelling it out) is a computer storage format that combines a file system based on the copy-on-write (COW) principle with a logical volume manager (not to be confused ...

and FreeBSD UFS2, attempt to solve this through techniques called block suballocation and tail merging. Other file systems such as ZFS support variable block sizes. Block storage is normally abstracted by a file system or database management system (DBMS) for use by applications and end users. The physical or logical volumes accessed via ''block I/O'' may be devices internal to a server, directly attached via SCSI or Fibre Channel, or distant devices accessed via a

storage area network A storage area network (SAN) or storage network is a computer network which provides access to consolidated, block-level data storage. SANs are primarily used to access data storage devices, such as disk arrays and tape libraries from ser ...

(SAN) using a protocol such as

iSCSI Internet Small Computer Systems Interface or iSCSI ( ) is an Internet Protocol-based storage networking standard for linking data storage facilities. iSCSI provides block-level access to storage devices by carrying SCSI commands over a TCP/IP ...

, or AoE. DBMSes often use their own block I/O for improved performance and recoverability as compared to layering the DBMS on top of a file system.

References

{{reflist, refs= {{cite news , author= , title=Available hard drive space, block sizes, and size terminology , newspaper=CNET , date=2009-05-05 , url=http://www.cnet.com/news/available-hard-drive-space-block-sizes-and-size-terminology/ , access-date=2014-04-29 {{cite web , author-last=Chang , author-first=S. K. , title=Physical Structures , work=Captain SK , url=http://people.cs.pitt.edu/~chang/156/08struct.html , access-date=2014-04-29 {{cite news , author-first=Rachel , author-last=Balik , title=Bruning Questions: ZFS Record Size , newspaper=Joyent , date=2013-03-29 , url=https://www.joyent.com/blog/bruning-questions-zfs-record-size/ , access-date=2013-03-29 {{cite news , author-first=Roch , author-last=Bourbonnais , title=Tuning ZFS recordsize , date=2006-06-07 , newspaper=Oracle , url=https://blogs.oracle.com/roch/tuning-zfs-recordsize {{citation , title=Planning a Computer System – Project Stretch , author-first1=Gerrit Anne , author-last1=Blaauw , author-link1=Gerrit Anne Blaauw , author-first2=Frederick Phillips , author-last2=Brooks, Jr. , author-link2=Frederick Phillips Brooks, Jr. , author-first3=Werner , author-last3=Buchholz , author-link3=Werner Buchholz , editor-first=Werner , editor-last=Buchholz , editor-link=Werner Buchholz , publisher= McGraw-Hill Book Company, Inc. / The Maple Press Company, York, PA. , lccn=61-10466 , date=1962 , chapter=4: Natural Data Units , pages=39–40 , chapter-url=http://archive.computerhistory.org/resources/text/IBM/Stretch/pdfs/Buchholz_102636426.pdf , access-date=2017-04-03 , url-status=live , archive-url=https://web.archive.org/web/20170403014651/http://archive.computerhistory.org/resources/text/IBM/Stretch/pdfs/Buchholz_102636426.pdf , archive-date=2017-04-03 , quote= ��Terms used here to describe the structure imposed by the machine design, in addition to ''

'', are listed below.
''

Byte The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable uni ...

'' denotes a group of bits used to encode a character, or the number of bits transmitted in parallel to and from input-output units. A term other than '' character'' is used here because a given character may be represented in different applications by more than one code, and different codes may use different numbers of bits (i.e., different byte sizes). In input-output transmission the grouping of bits may be completely arbitrary and have no relation to actual characters. (The term is coined from ''

bite Biting is a common zoological behavior involving the active, rapid closing of the jaw around an object. This behavior is found in toothed animals such as mammals, reptiles, amphibians and fish, but can also exist in arthropods. Myocytic co ...

'', but respelled to avoid accidental mutation to ''bit''.)
A ''

word A word is a basic element of language that carries an objective or practical meaning, can be used on its own, and is uninterruptible. Despite the fact that language speakers often have an intuitive grasp of what a word is, there is no conse ...

'' consists of the number of data bits transmitted in parallel from or to memory in one memory cycle. Word size is thus defined as a structural property of the memory. (The term '' catena'' was coined for this purpose by the designers of the

Bull A bull is an intact (i.e., not castrated) adult male of the species ''Bos taurus'' (cattle). More muscular and aggressive than the females of the same species (i.e., cows), bulls have long been an important symbol in many religions, includin ...

{{ill, Bull Gamma 60{{!GAMMA 60, fr, Gamma 60 computer.)
''Block'' refers to the number of words transmitted to or from an input-output unit in response to a single input-output instruction. Block size is a structural property of an input-output unit; it may have been fixed by the design or left to be varied by the program. ��} Computer data storage Data transmission