SUSE LINUX 9.3

Chapter 3. Special Installation Procedures / 3.7. LVM Configuration
	3.6. Permanent Device Names for SCSI Devices		3.8. Soft RAID Configuration

3.7. LVM Configuration

This section briefly describes the principles behind LVM and its basic features that make it useful under many circumstances. In Section 3.7.2, “LVM Configuration with YaST”, learn how to set up LVM with YaST.

	Warning
Using LVM might be associated with increased risk, such as data loss. Risks also include application crashes, power failures, and faulty commands. Save your data before implementing LVM or reconfiguring volumes. Never work without a backup.

3.7.1. The Logical Volume Manager

The Logical Volume Manager (LVM) enables flexible distribution of hard disk space over several file systems. 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 1.5.4.1, “Partition Types” and Section 2.7.5, “Partitioning”.

Figure 3.3. Physical Partitioning versus LVM

Figure 3.3, “Physical Partitioning versus LVM” 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.
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. This would allow file systems that are larger than the physical hard disk. Another advantage of LVM is that up to 256 LVs can be added. However, keep in mind that working with LVM is different from working with conventional partitions. Instructions and further information about configuring LVM is available in the official LVM HOWTO at http://tldp.org/HOWTO/LVM-HOWTO/.

Starting from kernel version 2.6, LVM version 2 is available, which is downward-compatible with the previous LVM and enables the continued management of old volume groups. When creating new volume groups, decide whether to use the new format or the downward-compatible version. LVM 2 does not require any kernel patches. It makes use of the device mapper integrated in kernel 2.6. This kernel only supports LVM version 2. Therefore, when talking about LVM, this section always refers to LVM version 2.

3.7.2. LVM Configuration with YaST

The YaST LVM configuration can be reached from the YaST Expert Partitioner (see Section 2.7.5, “Partitioning”). This professional partitioning tool enables you to edit and delete existing partitions and create new ones that should be used with LVM. There, create an LVM partition by first clicking ‘Create’+‘Do not format’ then selecting ‘0x8E Linux LVM’ as the partition identifier. After creating all the partitions to use with LVM, click ‘LVM’ to start the LVM configuration.

3.7.2.1. Creating Volume Groups

If no volume group exists on your system yet, you are prompted to add one (see Figure 3.4, “Creating a Volume Group”). It is possible to create additional groups with ‘Add group’, but usually one single volume group is sufficient. system is suggested as a name for the volume group in which the SUSE LINUX system files are located. The physical extent size defines the size of a physical block in the volume group. All the disk space in a volume group is handled in chunks of this size. This value is normally set to 4 MB and allows for a maximum size of 256 GB for physical and logical volumes. The physical extent size should only be increased, for example, to 8, 16, or 32 MB, if you need logical volumes larger than 256 GB.

Figure 3.4. Creating a Volume Group

3.7.2.2. Configuring Physical Volumes

Once a volume group has been created, the following dialog lists all partitions with either the “Linux LVM” or “Linux native” type. No swap or DOS partitions are shown. If a partition is already assigned to a volume group, the name of the volume group is shown in the list. Unassigned partitions are indicated with “--”.

If there are several volume groups, set the current volume group in the selection box to the upper left. The buttons in the upper right enable creation of additional volume groups and deletion of existing volume groups. Only volume groups that do not have any partitions assigned can be deleted. All partitions that are assigned to a volume group are also referred to as a physical volumes (PV).

Figure 3.5. Physical Volume Setup

To add a previously unassigned partition to the selected volume group, first click the partition then ‘Add Volume’. At this point, the name of the volume group is entered next to the selected partition. Assign all partitions reserved for LVM to a volume group. Otherwise, the space on the partition remains unused. Before exiting the dialog, every volume group must be assigned at least one physical volume. After assigning all physical volumes, click ‘Next’ to proceed to the configuration of logical volumes.

3.7.2.3. Configuring Logical Volumes

After the volume group has been filled with physical volumes, define the logical volumes the operating system should use in the next dialog. Set the current volume group in a selection box to the upper left. Next to it, the free space in the current volume group is shown. The list below contains all logical volumes in that volume group. All normal Linux partitions to which a mount point is assigned, all swap partitions, and all already existing logical volumes are listed here. ‘Add’, ‘Edit’, and ‘Remove’ logical volumes as needed until all space in the volume group has been exhausted. Assign at least one logical volume to each volume group.

Figure 3.6. Logical Volume Management

To create a new logical volume, click ‘Add’ and fill out the pop-up that opens. As for partitioning, enter the size, file system, and mount point. Normally, a file system, such as reiserfs or ext2, is created on a logical volume and is then designated a mount point. The files stored on this logical volume can be found at this mount point on the installed system. Additionally it is possible to distribute the data stream in the logical volume among several physical volumes (striping). If these physical volumes reside on different hard disks, this generally results in a better reading and writing performance (like RAID 0). However, a striping LV with n stripes can only be created correctly if the hard disk space required by the LV can be distributed evenly to n physical volumes. If, for example, only two physical volumes are available, a logical volume with three stripes is impossible.

	Striping
YaST has no chance at this point to verify the correctness of your entries concerning striping. Any mistake made here is apparent only later when the LVM is implemented on disk.

Figure 3.7. Creating Logical Volumes

If you have already configured LVM on your system, the existing logical volumes can be entered now. Before continuing, assign appropriate mount points to these logical volumes too. With ‘Next’, return to the YaST Expert Partitioner and finish your work there.

3.7.2.4. Direct LVM Management

If you already have configured LVM and only want to change something, there is an alternative way to do that. In the YaST Control Center, select ‘System’+‘LVM’. Basically this dialog allows the same actions as described above with the exception of physical partitioning. It shows the existing physical volumes and logical volumes in two lists and you can manage your LVM system using the methods already described.