Managing RAID on Linux

Logical Drives

It should be clear by now that sales sheets and documentation about RAID use many terms interchangeably, although their meaning varies depending on the context. The term logical drive is used to mean an array in some software RAID implementations. In the context of a controller, the term logical drive has another meaning. Some controllers let you split a single large array into multiple smaller logical drives. So, while I might create a single RAID-5 that is several hundred gigabytes in size, I can further segment that array by creating logical drives that contain a subset of the total storage space. This is useful when you are working with very large disks, but want to allot a manageable amount of space for system disks. It’s also useful if subpartitioning at the operating system level won’t work because you need more partitions than are supported by a single disk. Not all controllers implement logical drives.

Using logical drives also helps maximize storage space. Let’s say that you have a system with five 18 GB disks. You know already that you need roughly 25 GB to store a MySQL database. It might seem ideal to allocate two disks for a RAID-1 to house the system partitions. That leaves three remaining disks that you could use to create a RAID-5, yielding 36 GB of raw storage space. However, that’s not much room for database growth, and neither array has a spare disk. Furthermore, the system partitions on the RAID-1 aren’t going to use anywhere near the whole 18 ...