History
A "block", a contiguous number of bytes, is the minimum unit of memory that is read from and written to a disk by a disk driver. The earliest disk drives had fixed block sizes (e.g. the IBM 350 disk storage unit (of the late 1950s) block size was 100 6 bit characters) but starting with the 1301 IBM marketed subsystems that featured variable block sizes: a particular track could have blocks of different sizes. The disk subsystems on the IBM System/360 expanded this concept in the form of Count Key Data (CKD) and later Extended Count Key Data (ECKD); however the use of variable block size in HDDs fell out of use in the 1990s; one of the last HDDs to support variable block size was the IBM 3390 Model 9, announced May 1993
Modern hard disk drives such as Serial attached SCSI (SAS) and Serial ATA (SATA) drives, appear at their interfaces as a contiguous set of fixed-size blocks; for many years 512 bytes long but beginning in 2009 and accelerating through 2011, all major hard disk drive manufacturers began releasing hard disk drive platforms using the Advanced Format of 4096 byte logical blocks.
Floppy disks generally only used fixed block sizes but these sizes were a function of the host's OS and its interaction with its controller so that a particular type of media (e.g., 5ΒΌ-inch DSDD) would have different block sizes depending upon the host OS and controller.
Optical disks generally only use fixed block sizes.
Read more about this topic: Disk Formatting
Famous quotes containing the word history:
“What is most interesting and valuable in it, however, is not the materials for the history of Pontiac, or Braddock, or the Northwest, which it furnishes; not the annals of the country, but the natural facts, or perennials, which are ever without date. When out of history the truth shall be extracted, it will have shed its dates like withered leaves.”
—Henry David Thoreau (1817–1862)
“There are two great unknown forces to-day, electricity and woman, but men can reckon much better on electricity than they can on woman.”
—Josephine K. Henry, U.S. suffragist. As quoted in History of Woman Suffrage, vol. 4, ch. 15, by Susan B. Anthony and Ida Husted Harper (1902)
“This above all makes history useful and desirable: it unfolds before our eyes a glorious record of exemplary actions.”
—Titus Livius (Livy)