It’s common for servers to have multiple storage devices. A logical unit number (LUN) assigns a unique value to each drive. A LUN can be assigned to a group of drives configured as a single volume, a partition on a drive, or the entire drive itself. A LUN value can be automatically assigned or manually assigned by an administrator. In this article, we take a look at how LUNs work and the benefits and downsides of using them.
What Is a LUN in Storage?
A LUN (logical unit number) is a unique identifier that defines a storage partition in a storage area network (SAN) environment for data organization and access. It’s important to note that a LUN is a component in storage organization and not a type of storage device itself. A LUN is a numeric value that points to a physical disk or a logical set of disks. Storage LUNs can also point to a logical set of partitions.
The purpose of a LUN is for clients to make requests from storage space and retrieve data. Usually, LUNs refer to SAN storage, so client computers can map network drives, request data from network storage, or store data on the SAN. Users do not address a LUN when mapping a drive, so only administrators manage LUN identifiers.
How Does a LUN Work?
When you build a new server or add a drive to a server, you must partition it. Partitioning forces you to choose a file system. You can choose to make the entire drive space a part of the partition, use only some of the drive space for a partition, or create a logical volume from a group of drives. A LUN can be assigned to the new partition or a set of partitions.
When a server has several disks configured as a RAID (redundant array of independent disks) on SCSI (Small Computer System Interface) ports, the server uses a LUN to communicate with the right storage unit. LUN assignment is common with SCSI disks, but a storage area network (SAN) with other storage ports (e.g., SATA or SAS) could be configured as a RAID and connected via a Fibre Channel where a LUN is assigned.
For a user, the LUN is seen as a single-mounted storage device even if several partitions or a RAID of disks represent a single LUN. A server accessing internal storage needs the LUN to identify the right disk to read to or write from. LUNs assigned to a SAN are necessary for client computers to mount the right disk and assign it a name (e.g., X or Z as a drive letter).
It’s possible for a single LUN to identify thousands of logical volumes, especially in a SAN environment. Administrators can assign a LUN to SAN volumes or the SAN unit can automatically assign a LUN to storage. LUNs can be reassigned by administrators later after bootup to customize configurations to meet certain setup requirements.
Alternatives and Implementations of LUN Storage
Every server or network storage device has a LUN so that operating systems can identify a volume to read or write to disks. For many enterprise environments, large network storage devices use RAID storage, which means that several disks could be represented by a single LUN. A large SAN with several disks configured as a single volume could also have a single LUN.
Unless you use older SCSI technology on your personal computer, you only find LUN storage in older SAN or server hardware. Newer SATA (Serial Advanced Technology Attachment) drives are used in servers and personal computers, but SCSI was replaced with SAS (Serial Attached SCSI) or iSCSI (Internet Small Computer System Interface). Enterprise systems may use SATA or SAS depending on the type of storage and speeds necessary to support a large volume of read and write requests.
Benefits of Using LUN Storage
Although a LUN represents some or all of a disk’s storage capacity, it does not always represent a single disk. LUN management software lets administrators choose LUN values for specific disks or volumes, but counting the number of LUN storage values does not represent the total number of disks. The operating system on the remote computer using the LUN can mount it as a drive, so the storage can be given a user-friendly name, usually a letter (e.g., Z drive or X drive).
LUN management lets administrators better control SAN disks and capacity. Enterprise networks use several large-capacity drives to hold petabytes of data, and a LUN lets a workstation mount a single LUN as a user-friendly drive letter. These drive letters often have logical organization (e.g., storage for accounting or sharing documents company-wide) with user-friendly volume letters that most employees know when they transfer documents to network drives.
Although a LUN enables mounting drives and resource sharing on a local device, it does not offer any security. Administrators are still responsible for creating user groups and assigning appropriate permissions to each user group for data access control. Most organizations assign a different drive letter to each user group so there’s no confusion for employees about the purpose of each mounted drive. For example, the X drive might be used for sharing files with anyone within the organization, while the Z drive might represent a link to personal file spaces that only a single employee can access.
Potential Downsides of a LUN
A LUN is a numeric assignment for SCSI or a Fibre Channel, but a LUN is often associated with a SAN using a RAID system. A standard user will not run into LUN assignments, but administrators working with legacy hardware might find LUN management challenging. Remember that a LUN represents a slice of storage in a SAN, but it does not always represent a single disk or partition. LUN assignments can be manual or automatic, but most administrators use SAN software to manage LUN values.
Any downsides to LUN are management related. Initial setup might be somewhat convenient, but any new space added to the SAN must be given a LUN or added to existing volumes so that current storage capacity can be expanded. Some administrators might struggle to add new disks to a LUN depending on the operating system and SAN configuration. Most operating systems have an interface to make it somewhat convenient to add storage to current capacity, but administrators might find that they need to troubleshoot when new disks aren’t recognized by the system.
LUNs aren’t always assigned in a SAN or NAS environment. RAID volumes can also exist on a physical server where administrators can assign a LUN. An operating system running on a virtual machine can also use a LUN. Administrators should create a LUN strategy to ensure that reads and writes are optimized for performance. Too many applications and users writing to the same LUN can cause performance degradation, so ensure that LUNs are assigned with performance in mind.
Conclusion
Logical unit numbers (LUNs) play an indispensable role in the realm of block storage, serving as a pivotal link between physical storage resources and the data they hold. As we've explored, LUNs facilitate the efficient and flexible allocation of storage space, allowing for the dynamic management of data in various environments, from small-scale setups to large data centers.
Administrators working with enterprise solutions might find themselves working with LUN management, typically in a large SAN environment. For systems that use SCSI RAID environments, LUN assignment is a component in mounting drives on servers and client workstations.
Understanding LUNs is just one piece of knowledge crucial for anyone involved in storage administration or data management as it provides the foundational knowledge required for efficient storage utilization and data retrieval. The management and support of your storage infrastructure can consume precious IT resources. Learn how Pure Professional Services can help reduce the administrative overhead of managing your storage arrays.
