Data Storage Fundamentals: Understanding the Key Terms
Park Place Technologies
Today, we’re getting back to basics. LUNs, volumes, and partitions are three of the most fundamental concepts in data storage. If you’re fairly new to storage, you’ve probably heard these terms and wondered what they meant. So we’re going to break down just what they are and how they relate to each other.
LUNs (logical unit number) are critical for managing block storage over a SAN. A LUN is a logical abstraction between the physical disk or volume, and the applications running on it. The disk or volume could be a single drive, a partition of a single drive, or a volume from a RAID controller.
LUNs are used to present larger or smaller views of the disk to the server. As an example, if you partition a drive into smaller pieces, the pieces would still share a common SCSI target ID address with each partition being a unique LUN.
Another example would be a RAID controller where multiple disk drives are mirrored or striped with parity 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.
You don’t always need a LUN to use a disk device, however. If you are simply attaching JBOD, “just a bunch of disks,” to a server, LUNs may not have to be created. Also, 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.
A volume is a single accessible storage area within a filesystem, usually a hard drive or a partition of a drive. Even if a volume isn’t a physical disk, it can still be accessed with an operating system’s logical interface.
The concepts are similar, but a volume is not the same thing as a partition. For example, a floppy disk might be accessible as a volume, even though it does not contain a partition, since floppy disks cannot be partitioned with most modern computer software.
Also, an Operating System (OS) can recognize a partition without recognizing any volume associated with it, as when the OS cannot interpret the filesystem stored there. This occurs when Windows NT-based operating systems encounter disks with non-Microsoft partitions, such as the ext3 (third extended) filesystem commonly used with Linux.
In Windows systems, volumes are handled by the kernel and managed using the Disk Management MMC snap-in or the Diskpart command line tool, while Linux volumes are usually handled by the Logical Volume Manager or Enterprise Volume Management System.
Partitions are necessary because you can’t write files to a blank hard drive. Before they’re partitioned, a storage device just look like a mass of unallocated space to an operating system.
You have to create at least one container with a file system to begin writing to disk, and we call this container a partition. You can have one partition that contains all the storage space on the drive or divide up the space into multiple partitions.
After creating a partition, the partition is formatted with a file system — like the NTFS file system on Windows drives, FAT32 file system for removable drives, HFS+ file system on Mac computers, or the ext4 filesystem on Linux. Files are then written to that file system on the partition.
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