Logical block addressing: Encyclopedia II - Logical block addressing - The LBA scheme

Logical block addressing - The LBA scheme

LBA is a particularly simple addressing scheme; blocks are simply located by an index, with the first block being LBA=0, the second LBA=1, and so on. The LBA scheme replaces earlier schemes which exposed the physical details of the storage device to the software of the operating system. Chief among these was the cylinder-head-sector (CHS) scheme, where blocks were addressed by means of a tuple which defined the cylinder, head, and sector at which they appeared on the hard disk. CHS didn't map well to devices other than hard disks (such as tapes and networked storage), and was generally not used for them. CHS was used in early MFM and RLL drives, and it and its successor Extended Cylinder-Head-Sector (ECHS) were used in the first ATA drives.

SCSI introduced LBA as an abstraction. While the drive controller still addresses data blocks by their CHS address, this information is generally not used by the SCSI device driver, the OS, filesystem code, and any applications (such as databases) that access the "raw" disk. System calls requiring block-level I/O pass LBA definitions to the storage device driver; for simple cases (where one volume maps to one physical drive) then this LBA is passed directly to the drive controller.

Adapted from the Wikipedia article "The LBA scheme", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki