Recommended Free Tools
A logical unit number (LUN) is a 64-bit identifier that a SCSI host uses to address a logical unit inside a target device. A LUN is the address, not the storage object it points to, and it is not automatically a physical disk.
What the term means
The SCSI standards separate two ideas that people often merge. The logical unit is the thing being addressed: ISO/IEC 14776-453:2009 (section 3.1.58) defines it as “An externally addressable entity within a SCSI target device that implements a SCSI device model and contains a device server.” The logical unit number is the identifier for that entity: section 3.1.61 defines it as “An encoded 64-bit identifier for a logical unit.”
As an Amazon Associate I earn from qualifying purchases.
In practice, a host sends a command to a target device and includes a LUN to say which logical unit should process it. The LUN is the label; the logical unit is what receives the command.
Do these 3 things before closing this tab:
1Scan for outdated or missing drivers - takes under a minute2Repair Windows errors before they cause bigger problems3Fix the driver behind crashes, sound loss and screen glitchesThe parts of the model
| Term | What it is | Where the definition comes from |
|---|---|---|
| Target device | The SCSI device that contains logical units and target ports | ISO/IEC 14776-453:2009; RFC 4455 (2006) |
| Logical unit (LU) | An addressable entity that implements a device model and contains a device server, which processes SCSI commands | ISO/IEC 14776-453:2009, section 3.1.58 |
| Logical unit number (LUN) | The encoded 64-bit identifier used to select a logical unit | ISO/IEC 14776-453:2009, section 3.1.61; RFC 4455 (2006) |
| Physical storage | The underlying hardware or media that may back a logical unit | Not required by the definitions to map one-to-one to a logical unit |
How a LUN is used in iSCSI
iSCSI carries SCSI commands over TCP/IP, so the same addressing idea applies on the network. RFC 7143 (2014), section 11.2.1.7, states: “Some Opcodes operate on a specific LU. The Logical Unit Number (LUN) field identifies which LU.” The field is 64 bits and is to be formatted in accordance with SAM-2, the SCSI Architecture Model standard that RFC references.
#1 Best Overall
- IronWolf internal hard drives are the ideal solution for up to 8-bay, multi-user NAS environments craving powerhouse performance
- Store more and work faster with a NAS-optimized hard drive providing ultra-high capacity up to 16TB and cache of up to 256MB
- Purpose built for NAS enclosures, IronWolf delivers less wear and tear, little to no noise/vibration, no lags or down time, increased file-sharing performance, and much more
- Easily monitor the health of drives using the integrated IronWolf Health Management system and enjoy long-term reliability with 1M hours MTBF
- Three-year limited warranty protection plan included and three year Rescue Data Recovery Services included
When you troubleshoot an iSCSI connection, the LUN is the part of the command that tells the target which unit to act on. A host that sees a target but no expected LUN is usually dealing with a mapping, presentation or access question on the storage side, not with a change to the definition itself.
A LUN is not automatically a physical disk
This is the most common misunderstanding. The standards define a LUN as an identifier for a logical unit. They do not say that every logical unit must correspond to a single physical disk, and they do not say that one LUN always equals one drive or one volume.
Rank #2
- Store more, compute faster, and do it confidently with the proven reliability of BarraCuda internal hard drives
- Build a power house gaming computer or desktop setup with a variety of capacities and form factors
- The go to SATA hard drive solution for nearly every PC application from music to video to photo editing to PC gaming. Ax. Sustained transfer rate OD: 190MB/s
- Confidently rely on internal hard drive technology backed by 20 years of innovation
- Frustration Free Packaging - This is just an anti-static bag. No cables, no box.
A logical unit may be backed by a single disk, part of a disk, a RAID set, or storage spread across several devices, depending on how the storage system is built. Which of these applies is a property of the particular array or software, not of the LUN concept. To find out how a specific system maps its logical units to physical media, check that vendor’s current product documentation.
Key facts to keep
- The logical unit is the addressable entity; the LUN is its identifier.
- The LUN is a 64-bit value (ISO/IEC 14776-453:2009; RFC 4455, 2006).
- The logical unit sits inside a SCSI target device alongside target ports.
- In iSCSI, the LUN field tells the target which logical unit a command addresses (RFC 7143, 2014).
- A LUN does not, by the standards’ definitions, equal a particular physical disk.
Scope of these references
The definitions above come from SCSI and iSCSI standards, not from any one vendor. The 64-bit width describes the identifier format; it is a specification, not a measure of how widely LUNs are used. The standards do not describe how any particular product presents, numbers or maps its logical units, so treat vendor-specific setup as a separate question.
Quick Recap
Rank #4
- Available in capacities ranging from 2 to 22TB(1) | (1) 1GB = 1 billion bytes and 1TB = 1 trillion bytes. Actual user capacity may be less depending on operating environment.
- For RAID-optimized NAS systems with unlimited number of bays
- Rated for 550TB/yr workload rate(2) | (2) Annualized Workload Rate = TB transferred x (8760 / recorded power-on hours). The maximum rated workload is specified for operating at typical temperature of 40C. Workload Rate will vary depending on your hardware and software components and configurations.
- Designed to handle the demands of high-intensity 24x7 multi-user NAS environments
- Western Digital partners with a wide range of NAS system vendors for extensive testing to ensure compatibility with most NAS enclosures
Rank #3
- IronWolf internal hard drives are the ideal solution for up to 8-bay, multi-user NAS environments craving powerhouse performance.date transfer rate:6.0 gigabits_per_second
- Store more and work faster with a NAS-optimized hard drive providing 8TB and cache of up to 256MB
- Purpose built for NAS enclosures, IronWolf delivers less wear and tear, little to no noise/vibration, no lags or down time, increased file-sharing performance, and much more
- Easily monitor the health of drives using the integrated IronWolf Health Management system and enjoy long-term reliability with 1M hours MTBF
- Three-year limited product warranty protection plan and three year Rescue Data Recovery Services included
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.
