It was developed because sometimes the need to change the segmentation of hard disk space arises only after the initial partitioning during installation has already been done. Because it is difficult to modify partitions on a running system, LVM provides a virtual pool (volume group, VG for short) of memory space from which logical volumes (LVs) can be created as needed. The operating system accesses these LVs instead of the physical partitions. Volume groups can span more than only one disk so that several disks or parts of them may constitute one single VG. This way, LVM provides a kind of abstraction from the physical disk space that allows its segmentation to be changed in a much easier and safer way than physical repartitioning does. Background information regarding physical partitioning can be found in Section “Par- 58 tition Types” (Chapter 1, Installation with YaST, ?Start-Up) and Section “Partitioner” (Chapter 3, System Configuration with YaST, ?Start-Up). Figure 2.1 Physical Partitioning versus LVM PART PART PART PART PART DISK PART PART PART MP MP MP MP MP MP MP DISK 1 DISK 2 VG 1 VG 2 LV 1 LV 2 LV 3 LV 4 Figure 2.1, “Physical Partitioning versus LVM” (page 59) compares physical partitioning (left) with LVM segmentation (right). On the left side, one single disk has been divided into three physical partitions (PART), each with a mount point (MP) assigned so that the operating system can access them. On the right side, two disks have been divided into two and three physical partitions each. Two LVM volume groups (VG 1 and VG 2) have been defined. VG 1 contains two partitions from DISK 1 and one from DISK 2. VG 2 contains the remaining two partitions from DISK 2. In LVM, the physical disk partitions that are incorporated in a volume group are called physical volumes (PVs). Within the volume groups, four logical volumes (LV 1 through LV 4) have been defined, which can be used by the operating system via the associated mount points. The border between different logical volumes need not be aligned with any partition border. See the border between LV 1 and LV 2 in this example. LVM features: • Several hard disks or partitions can be combined in a large logical volume. • Provided the configuration is suitable, an LV (such as /usr) can be enlarged when the free space is exhausted. • Using LVM, even add hard disks or LVs in a running system. However, this requires hot-swappable hardware that is capable of such actions. Advanced Disk Setup 59 • It is possible to activate a “striping mode” that distributes the data stream of a logical volume over several physical volumes. If these physical volumes reside on different disks, this can improve the reading and writing performance just like RAID 0. • The snapshot feature enables consistent backups (especially for servers) in the running system. With these features, using LVM already makes sense for heavily used home PCs or small servers. If you have a growing data stock, as in the case of databases, music archives, or user directories, LVM is just the right thing for you.
