LUN storage: Working with a SAN's logical unit numbers
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A reader recently asked: What is a LUN and why do we have to create one rather than use the disk as it is? A logical...
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unit number (LUN) can refer to an entire physical disk, or a subset of a larger physical disk or disk volume. The physical disk or disk volume could be an entire single disk drive, a partition (subset) of a single disk drive, or a disk volume from a redundant array of independent disks (RAID) controller comprising multiple disk drives aggregated together for larger capacity and redundancy. LUNs are an important factor in managing block storage arrays shared over a SAN. A LUN represents a logical abstraction or, if you prefer, virtualization layer between the physical disk device/volume and the applications.
The term virtualization has been used to mean many things; thus, the virtualization police may or may not agree to a LUN being a layer of virtualization. A LUN can be used to present a larger or smaller view of the disk storage to the server. For example, if you partition a disk drive into smaller pieces for your application or system needs (perhaps your server's operating system has a disk drive size limit) the sub-segments would share a common Small Computer System Interface (SCSI) target ID address with each partition being a unique LUN.
Another example would be a RAID controller where multiple disk drives are mirrored (RAID 1) or stripped with parity (RAID 5) to create a larger physical disk volume. A LUN would be used to represent the virtual disk devices made up of the disks in the RAID group. So, you may not need to create a LUN to use a disk device. For example, if you are simply attaching several JBOD disks to a server via SCSI, USB drive, Fibre Channel or whatever interface you prefer, LUNs may not have to be created. If you are using a SCSI disk and you have not subdivided it into partitions, the device would show up with a unique SCSI ID target and default LUN 0. Note that you will want to ensure the devices are properly addressed to avoid conflicts.
When it comes down to deciding whether or not you need a LUN, you have to ask yourself what type of storage you have, how it is configured and how you will use it. Most likely, unless you are using JBOD or a single disk device, you will use LUNs as an abstraction layer to access your storage.
LUN zoning and LUN masking
Now that we've answered the question "What is a LUN?" the next step is to discuss security features that will ensure users and servers see only those storage resources they are allowed to access.
Logical unit number zoning isolates a single server to a specific logical unit or associates a group of servers with a logical unit. LUN zoning also enables a storage administrator to allocate bandwidth by assigning particular ports to a zone.
Once LUN zoning is completed, the LUN masking process subdivides port access so that only the logical unit numbers authorized to access a specific server can access the corresponding port.
LUN creation for storage-area networks
Five steps to managing LUNs
The role of LUNs in virtual environments
Randy Kerns asks:
Will there ever come a time when computing environments will be LUN-free?
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