A direct-access storage device (DASD) pronounced /ˈdæzdiː/) 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 minicomputers. Disk drives, magnetic drums, data cells and optical disc drives are all classified as DASDs.
Contents
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 channels, a type of subordinate mini-processor. Channel programs write to, read from, and control the given device.
CTR (CHR)
The operating system uses a four byte relative track and record (TTR) for some access methods and for others an eight-byte extent-bin-cylinder-track-record block address, or MBBCCHHR, Channel programs address DASD using a six byte seek address (BBCCHH) and a five byte record identifier (CCHHR).
When the 2321 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 track, but some devices had a track overflow feature that allowed breaking a large block into track-size segments within the same cylinder.
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 CKD, which refers to Count Key Data, 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/370 and DOS/VSE, but not MVS or successor operating systems in the OS/360 line.
Access
The programming interface macros and routines are collectively called DAM: direct access methods.
DOS/VSE
MVS, OS/390
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.