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Logical block addressing (LBA) is a method for identifying storage locations with a linear sequence of numbers. Address 0 identifies the first logical block, address 1 the next, and the sequence continues through the device or partition’s final block. An LBA is an address—not a promise that the block is 512 bytes or that it corresponds one-to-one with a physical sector.
What does logical block addressing mean?
LBA stands for logical block addressing. The ATA-3 glossary defines it as addressing a device through a linear mapping of sectors. In practical terms, software can refer to a block by its position in an ordered sequence instead of describing its location with several geometry-style values. ATA-3, section 2.1.11
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In the SCSI-2 model, numbering starts at block zero and advances contiguously to the last logical block on a logical unit or within a partition. The exact addressing fields and limits depend on the protocol and command being used. SCSI-2 specification
How is LBA different from CHS?
CHS means cylinder, head, and sector. It describes a location with three geometry-style values; LBA represents the location with one linear block number. A legacy Seagate ATA manual illustrates the mapping by identifying LBA 0 with cylinder 0, head 0, sector 1. That is a historical example of translating between representations, not a requirement for contemporary software to work out a disk’s physical geometry. Seagate ATA manual
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Is an LBA the same as a sector?
Not exactly. An LBA identifies a logical block; the relevant device or protocol specifies that block’s logical length. “Sector” is common terminology in ATA contexts, but a logical block’s size and the physical media’s sector organization are not necessarily the same.
The ATA/ATAPI Command Set-3 draft, Revision 5 (2013), describes both drives with 512-byte logical sectors grouped over 4,096-byte physical sectors and drives with native 4,096-byte logical and physical sectors. These are examples of possible layouts, not a description of every drive. ATA/ATAPI Command Set-3 draft, Revision 5
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To convert an LBA to a byte offset, multiply it by the applicable logical block size:
byte offset = LBA × logical block size
For example, the calculation must use the device’s reported logical block length; assuming 512 bytes without checking can produce the wrong offset.
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Why does sector alignment matter?
When a device uses larger physical sectors than logical sectors, a write that starts or ends at an LBA misaligned with the physical-sector boundary may require a read-modify-write operation. The device reads the larger physical sector, changes the requested portion, and writes the sector back. The ATA/ATAPI Command Set-3 draft documents this as a possible behavior; it does not establish a universal performance penalty or guarantee that every misaligned write will behave identically. ATA/ATAPI Command Set-3 draft, Revision 5
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Does LBA have one fixed maximum address?
No single maximum applies to every LBA system. The address width depends on the protocol and command format. For example, the older SCSI-2 specification describes a 21-bit LBA field in a six-byte command descriptor block and 32-bit fields in ten- and twelve-byte command descriptor blocks. Those are examples from that specification, not limits for every current storage interface. SCSI-2 specification
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LBA is an addressing scheme. It does not itself add storage capacity or guarantee a particular level of performance; those depend on the device and the interface’s specific implementation.
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