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 8.5.7.1, “Partition Types” and Section 8.5.7, “Using the YaST Partitioner”.

Figure 7.1. Physical Partitioning versus LVM¶

Figure 7.1, “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:

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.

Instead of LVM 2, you can use EVMS (Enterprise Volume Management System), which offers a uniform interface for logical volumes and RAID volumes. Like LVM 2, EVMS makes use of the device mapper in kernel 2.6.

The YaST LVM configuration can be reached from the YaST Expert Partitioner (see Section 8.5.7, “Using the YaST Partitioner”). This 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.

7.1.2.1. Creating Volume Groups¶

If no volume group exists on your system yet, you are prompted to add one (see Figure 7.2, “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 Enterprise® 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 7.2. Creating a Volume Group¶

7.1.2.2. 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 7.3. 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.

7.1.2.3. 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 7.4. 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 7.5. 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.

The Enterprise Volume Management System 2 (EVMS2) is a rich, extensible volume manager with built-in cluster awareness. Its plug-in framework allows plug-ins to add functionality for support and knowledge of any partition type. Being cluster-aware, EVMS2 guarantees that managed devices are named identically on each node in the cluster for easier management.

EVMS2 provides a unified interface (evmsgui and command line) for managing the following storage resources:

In SUSE Linux Enterprise Server 10, new features include the following:

SUSE® Linux Enterprise offers the option of combining several hard disks into one soft RAID system with the help of YaST—a very reasonable alternative to hardware RAID. RAID implies several strategies for combining several hard disks in a RAID system, each with different goals, advantages, and characteristics. These variations are commonly known as RAID levels.

Common RAID levels are:

The YaST soft RAID configuration can be reached from the YaST Expert Partitioner, described in Section 8.5.7, “Using the YaST Partitioner”. This partitioning tool enables you to edit and delete existing partitions and create new ones that should be used with soft RAID. There, create RAID partitions by first clicking Create+Do not format then selecting 0xFD Linux RAID as the partition identifier. For RAID 0 and RAID 1, at least two partitions are needed—for RAID 1, usually exactly two and no more. If RAID 5 is used, at least three partitions are required. It is recommended to take only partitions of the same size. The RAID partitions should be stored on different hard disks to decrease the risk of losing data if one is defective (RAID 1 and 5) and to optimize the performance of RAID 0. After creating all the partitions to use with RAID, click RAID+Create RAID to start the RAID configuration.

In the next dialog, choose between RAID levels 0, 1, and 5 (see Section 7.2.1, “RAID Levels” for details). After Next is clicked, the following dialog lists all partitions with either the “Linux RAID” or “Linux native” type (see Figure 7.6, “RAID Partitions”). No swap or DOS partitions are shown. If a partition is already assigned to a RAID volume, the name of the RAID device (for example, /dev/md0) is shown in the list. Unassigned partitions are indicated with “--”.

Figure 7.6. RAID Partitions¶

To add a previously unassigned partition to the selected RAID volume, first click the partition then Add. At this point, the name of the RAID device is entered next to the selected partition. Assign all partitions reserved for RAID. Otherwise, the space on the partition remains unused. After assigning all partitions, click Next to proceed to the settings dialog where you can fine-tune the performance (see Figure 7.7, “File System Settings”).

Figure 7.7. File System Settings¶

As with conventional partitioning, set the file system to use as well as encryption and the mount point for the RAID volume. After completing the configuration with Finish, see the /dev/md0 device and others indicated with RAID in the expert partitioner.

Check the file /proc/mdstats to find out whether a RAID partition has been destroyed. In the event of a system failure, shut down your Linux system and replace the defective hard disk with a new one partitioned the same way. Then restart your system and enter the command mdadm /dev/mdX --add /dev/sdX. Replace 'X' with your particular device identifiers. This integrates the hard disk automatically into the RAID system and fully reconstructs it.

Configuration instructions and more details for soft RAID can be found in the HOWTOs at:

Linux RAID mailing lists are also available, such as http://marc.theaimsgroup.com/?l=linux-raid&r=1&w=2.