A LUN can represent one disk, an entire redundant array of independent disks (RAID), or partitions of a disk, all of which execute I/O commands. LUNs allow users to differentiate between and manage separate shared volumes on a single SAN. They are the identifiers for building blocks of information on a physical disk drive and in some cases, virtual drives or virtual machines (VMs). LUNs are used to label slices of disk storage that are viewable from a server. They can also function as partitions, sectioning off portions of a volume from one another. They separate portions of disks that use different operating systems or have unique application requirements. Today, virtual or “thin” LUNs are provisioned on virtual disks, representing virtual storage with no association to storage on any physical drive, disk or device.
LUNs can be used in a variety of ways, and therefore can be broken into about five major categories that define some different uses.
A simple LUN is the basic building block upon which other types are based. A simple LUN represents one portion of one disk or one physical disk in its entirety—that's it. On the other hand, some LUNs are larger than one physical disk, so they “span” across two or more physical disks; these are called spanned LUNs.
Mirrored LUNs do use two physical disks but only for mirroring the information and data held within one of the disks. The other disk “reflects” what is held on the first in the event the first disk is harmed or experiences any sort of failure. Operations are performed simultaneously; so, it isn’t just the information on the physical disk that’s the same. The actions performed are also identical from the first disk to the mirrored disk.
The striped LUN also uses two or more disks in the same way as a spanned LUN., However, writing and reading is done on all disks at once using one spindle per disk, which enhances reading and writing times.
Striped LUNs with parity offer the same convenience as the striped LUN with the safety of backup data (parity) written to physical disks simultaneously. This protects against damage or failure. While LUNs striped with parity can significantly ease the mind because users don’t have to worry about losing information, creating parity takes a lot more time and space. Striping with parity is slower and therefore not ideal for some applications.
To manage a LUN, you must first be able to find it. That means finding the actual number that refers to the logical unit you’re trying to locate. Typically they are usually and most easily located by way of scanning the small computer system interface (SCSI). The system wizard or SCSI initiator combines the LUN with a port identifier to reveal which logical unit (or volume) you’ve discovered, which is seen as (and should be treated as) one device.
If you plan on creating your own LUNs, it’s imperative that the volumes you plan on partitioning and identifying with them are already configured. Once this is done but before creating any LUNs, you need to know the operating system (OS) they will utilize. The host OS can’t be changed once LUNs are created. There’s really no minimum size that a logical unit can be, but the maximum size recognized by most systems is 16TB. You’ll want to keep that in mind if you plan on extending your LUNs later or if you’re setting up a RAID-5 LUN that will contain voluminous data. Once you’ve managed this preliminary work you can move on to creation and mapping using the “1un” command system. This system identifies, names and maps LUNs across as many volumes as you like.
Resizing is as simple as finding the “lun resize” command within your system, as long as the volume it’s on has the space you need. LUNs can be extended up to 10 times their original size (as long as this does not surpass the 16TB maximum LUN size recognized by most systems). It’s important to understand that extending does not always increase the size and it never increases the size of the volume the LUN is on. An extension never works in the way an original LAN works because it is split by design. This can mean hiccups in performance and speed.
And, extending a LUN connected to a server won’t automatically mean more storage space for that server or the overall SAN. This is because creating extended LUNs doesn’t automatically create partitions of the same length as the newly extended LUN. Knowing this ahead of time will serve admins and users well, especially those responsible for creating, deleting or editing LUNs. Always follow the golden rule of LUN creation: Start with the size you anticipate needing later rather than creating smaller LUNs so you can have more of them or use less space.
Deleting a LUN is not the same thing as unmounting or detaching it. Deleting means writing over or otherwise erasing it so that the space on the volume can be unmounted or reformatted for other use. Unmounting or detaching means the logical unit number will still exist and still only function on the OS used to create it. The unmounted or detached LUN has simply been taken out of rotation on the server and SAN it’s mapped to (unmap).
Only logged-in admins should be able to delete a LUN. While OSs do vary some, the basics of LUN deletion are typically straightforward. Simply head to “LUN management” or a corresponding tool within your system. Next, have the system list the LUNs or find the list within the dropdown menu. Find executable actions within your menu and click “Delete LUN.” Most OSs will ask you again to be sure you want to delete it. Click “OK.”
If you’re unsure whether you want to delete a LUN, consider keeping it for a while but making it invisible. That way, if you change your mind later it will still be there. To make it invisible, unassign it using your OS’s LUN management tool. If you decide to make it visible again later, simply go to LUN management, find the invisible LUN in the list, and assign it once again.
One of the most important things you’ll do when creating or allocating LUNs will be to ensure that administrators can access the bandwidth they need. The way you do this—through LUN zoning—is also one of the ways you’ll keep your volumes and servers secure. Zoning is the practice of isolating an independent server using one LUN or pairing up groups of servers using the same LUN. When you have zoned your servers using LUNs, you can assign different users to the different zones. In turn, these users can assign new and different ports to the zone you’ve given them access to, which increases space for storage capabilities.
After zoning, masking divides access so only approved LUNs can come in via ports with matching values so no unauthorized users or ports will have access to your servers or storage. When you combine zoning and masking (no, they are not the same thing), you create a more secure environment for users and for the ports used on your system.
For more sophisticated security, try a fibre channel switch, which offers a more refined zoning system for servers using LUNs. Fibre channel switch systems permit only specific servers to access specific ports and other storage. Users of the fibre channel switch system choose it for enhanced security, which is why we discuss it in this section. They also use it because it is easily expanded and can therefore grow with your enterprise. In addition, fibre channel switch systems offer masking directly at the port for even more secure coming-and-going of information. And finally, for larger SANs, fibre channel switch systems can handle a multitude of servers.
In many respects the LUN has served its purpose well—just review all the above information and you’ll see this is true. However, LUNs will only take us so far, and the one place they cannot take us with security, longevity or quality scalability is cloud. Depending on agility, speed and ease of use, public and private cloud systems and other virtual spaces are quickly converting to VMs and containers that make them faster, smaller, cheaper to run and far more competitive in this space. Tintri Enterprise Cloud runs exclusively on VMs and containers to ensure you get the best of what you need at a lowered cost and performance. We’ll have your venture thriving.