LUN basics

For open systems environments, including Windows, fixed-block architectures or fixed-block addressing is the most common basis for performing I/O operations to and from a disk drive. The most common I/O command protocol for open systems is the SCSI command set, not to be confused with parallel SCSI cabling, as there is a difference. The SCSI command set is implemented on different network and storage I/O interfaces or transports, such as Fibre Channel, iSCSI, SAS, InfiniBand and others.

Part of the SCSI command set protocol is an initiator (source) that requests an I/O operation (read, write or status inquiry) and a target (destination) device with a subaddress known as a LUN. The target can be a single disk in a JBOD array with each HDD having a different SCSI target and possibly a unique or selectable LUN of the device supports it. A common use of LUNs is in storage systems or arrays that incorporate some type of controller, usually with RAID where multiple HDDs are configured and aggregated into a RAID or volume group and then assigned a unique LUN and accessed via a SCSI target ID.

For example, assume you have a storage system with 32 300 GB HDDs configured into two separate RAID groups, each with 16 HDDs. One RAID group is a RAID 5 14+1 and a hot spare, the other being a RAID 10 (8+8). The RAID 5 group would have about 4.2 TB of usable capacity, while the RAID 10 group would be about 2.4 TB. The storage system could present a single LUN or volume to a server or servers for each of the RAID groups or, the RAID groups could be subdivided into multiple smaller LUNs for more granular access of the storage depending on specific application needs or preferences.

About the author: Greg Schulz is founder and senior analyst with the IT infrastructure analyst and consulting firm StorageIO Group. Greg is also the author of Resilient Storage Networks (Elsevier) and a contributor to Storage magazine and other TechTarget venues.

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