Logical block addressing (LBA) is a common scheme used for specifying the location of blocks of data stored on computer storage devices, generally secondary storage systems such as hard disks. The term LBA can mean either the address or the block to which it refers. Logical blocks in modern computer systems are typically 512 or 1024 bytes each. ISO 9660 CDs (and images of them) use 2048-byte blocks.
The LBA scheme
LBA is a particularly simple linear addressing scheme; blocks are 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 both 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, or 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), this LBA is then passed directly to the drive controller.
LBA mapping and LUN virtualization
For more complex cases (particularly RAID devices and SANs and where logical drives (LUNs) are composed via LUN virtualization and aggregation), LBAs are translated from the application's model of the disk to that used by the actual storage device. In complex deployments, particularly when a storage fabric is employed, several of these LBA translations may occur between the dispatching application and the final, remote, disk.
Conversion between CHS to LBA
CHS addresses can be converted to LBA addresses using the following formula,
where,
- C, H and S are the cylinder number, the head number, and the sector number
- LBA is the logical block address
- HPC is the number of heads per cylinder
- SPT is the number of sectors per track
LBA addresses can be mapped to CHS address using the following formulae:
where
- mod is the modulo operation, i.e. the remainder, and
is integer division, i.e. the quotient of the division.
However, note that current disk drives use Zone Bit Recording, where the number of sectors per track depends on the track number. The disk drive will report a SPT to provide for these calculations, but which has little to do with the disk drive's true geometry.
Another formula:
LBA / spt = Result1 + Remainder1
S = Remainder1 + 1
Result1 / heads = Result2 + Remainder2
C = Result2
H = Remainder2
Example:
CHS = (600, 10, 84). Finding CHS for LBA = 1234 would be:
1234 / 84 = 14 R 58
S = 58 + 1 = 59
14 / 10 = 1 R 4
C = 1
H = 4
CHS = (1, 4, 59)
Test: ((1 * 10) + 4) * 84 + 59 - 1 = 14 * 84 + 58 = 1234
LBA, ATA devices and Enhanced BIOS
The first formal definition of the ATA standard interface allowed for 28 bit block addresses, using either LBA or CHS . The CHS mode used the 28 bits as follows: 16 bits of cylinder, 4 bits of head and 8 bits of sector. Note that CHS schemes count sectors from 1. Therefore the maximum number of sectors is 255 by this standard.
At the time of ATA-1 introduction, the BIOS standard for CHS was 24 bits, with 10 bits cylinder, 8 bits head, and 6 bits sector. This was defined by the BIOS INT 13H routine, and is used in the DOS style Master Boot Record. This created a need for translation between BIOS CHS and ATA CHS numbering, else only the least common denominator of 10/4/6 bits of CHS (respectively) could be used, or 1024*16*63 sectors for a limit of 504 MiB, assuming 512 byte sectors. Translation methods include the Large or Enhanced BIOS (also called Bit Shift Translation) which remapped the reported number of cylinders and heads, but left the sector numbers alone, and LBA-assist, which converted between the CHS systems by mapping first to LBA addressing.
At best, the 10/8/6 scheme of the BIOS meant an MS-DOS volume (and a Windows NT 4.0 system partition) was limited to 7.8 GiB, even given these methods of translation. The ATA-1 standard of 28 bit LBA limits disks to 128 GiB, assuming 512 byte sectors. In 2002 ATA-6 introduced the 48 bit LBA, extending the possible disk sizes to 128 PiB, assuming 512 byte sectors.
See also
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