Managing RAID on Linux

RAID on Linux

It’s important to understand that when I refer to a RAID array, I’m talking about a block device and not a filesystem. You could think of the relationship between the two much in the same way you might think of the relationship between a house and its foundation. If the foundation is weak, the house will eventually collapse. The filesystem, which represents the house in my analogy, is built on top of a block device. Normally, a block device is a single hard disk, but RAID introduces another layer (see Figure 1-5). RAID groups many block devices into a single virtual device.

Figure 1-5. Filesystems are built on block devices; RAID introduces an intermediary layer.

This means that Linux interacts with an array through a single block device having a single major and minor number. Physically, the array device points to many different physical disks, each with their own major and minor numbers. Programmers might think of this model the same way they think of an array data type, hence the use of the word “array” in the RAID acronym.

Each piece of hardware connected to a Linux system is assigned a major and minor number. The major number refers to a specific group of hardware (such as small computer systems interface, or SCSI, disks), while the minor uniquely identifies each installed piece of hardware within the group (for example, each individual SCSI disk). Since RAID is merely ...