Direct-access storage device

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A direct-access storage device (DASD) is a secondary storage device in which "each physical record has a discrete location and a unique address." IBM developed DASDs for use with mainframe computers and some minicomputer. Disk drives, drum memory, IBM 2321 Data Cell and optical disc drives are all classified as DASDs.

Access methods for DASDs include sequential, indexed, and direct. Direct access contrasts with the sequential access method used in tape drives. A record on a DASD can be accessed without having to read through intervening records from the current location, whereas reading anything other than the "next" record on tape requires skipping over intervening records, and requires a proportionally long time to access a distant point in a medium.

The DASD storage class includes both fixed and removable media.

Architecture

IBM mainframes access I/O devices including DASDs through Channel I/O, a type of subordinate mini-processor. Channel programs write to, read from, and control the given device.

CTR (CHR)

Channel programs address DASD using an eight-byte absolute module-bin-cylinder-track-record block address, or MBBCCHHR:

  • MBB representing the Module and Bin (for data cells),
  • CC representing the Cylinder,
  • HH representing the Head (or track), and
  • R representing the Record (block) number.

When the data cell was discontinued in January 1975, the addressing scheme and the device itself was referred to as CHR or CTR for cylinder-track-record, as the bin number was always 0.

IBM refers to the data records programmers work with as logical records, and the format on disc as blocks or physical records. One block might contain several logical (or user) records or, in some schemes, partial logical records.

Physical records can have any size up to the limit of a cylinder, although in usual practice, blocks or physical records do not exceed the capacity of a single track.

The access methods are responsible for blocking and deblocking logical records as they are written to or read from external media.

CKD

CHR/CTR acronyms should not be confused with Count Key Data (CKD), the layout of an addressable data record on a CTR.

FBA

In the 1970s, IBM introduced fixed block architecture (FBA) for mainframes. At the programming level, these devices do not use the traditional CHR addressing, but reference fixed-length blocks by number, much like sectors in mini-computers. More correctly, the application programmer remains unaware of the underlying storage arrangement, which stores the data in fixed physical block lengths of 512, 1024, 2048, or 4096.

For some applications, FBA not only offers simplicity, but an increase in throughput. FBA is supported by VM and VSE, but not MVS or successor operating systems in the OS/360 line.


Present terminology

Both drums and data cells have disappeared as products, so DASD remains as a synonym of disk and optical devices. Modern DASD used in mainframes only very rarely consist of single disk-drives. Most commonly "DASD" means large disk arrays utilizing RAID schemes.

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