Chapter 7. Logical Volume Manager (LVM)

7.1. What is LVM?

LVM is a method of allocating hard drive space into logical volumes that can be easily resized instead of partitions.

With LVM, a hard drive or set of hard drives is allocated to one or more physical volumes. A physical volume cannot span over more than one drive.

The physical volumes are combined into logical volume groups, with the exception of the /boot/ partition. The /boot/ partition cannot be on a logical volume group because the boot loader cannot read it. If the root (/) partition is on a logical volume, create a separate /boot/ partition which is not a part of a volume group.

Since a physical volume cannot span over multiple drives, to span over more than one drive, create one or more physical volumes per drive.

Figure 7-1. Logical Volume Group

The logical volume group is divided into logical volumes, which are assigned mount points, such as /home and /m and file system types, such as ext2 or ext3. When "partitions" reach their full capacity, free space from the logical volume group can be added to the logical volume to increase the size of the partition. When a new hard drive is added to the system, it can be added to the logical volume group, and partitions that are logical volumes can be expanded.

Figure 7-2. Logical Volumes

On the other hand, if a system is partitioned with the ext3 file system, the hard drive is divided into partitions of defined sizes. If a partition becomes full, it is not easy to expand the size of the partition. Even if the partition is moved to another hard drive, the original hard drive space has to be reallocated as a different partition or not used.

LVM support must be compiled into the kernel, and the default Red Hat kernel is compiled with LVM support.

To learn how to configure LVM during the installation process, refer to Chapter 8 LVM Configuration.