Fair Serving

Conventional partitioning is reaching its limits with the increasing storage capacity of hard disks and solid-state drives (SSDs). If several physical mass storage devices are attached to clusters or larger standalone servers, you can manage them more conveniently with the Linux Logical Volume Manager (LVM). Even small servers or single-user systems benefit from LVM if you are planning to change a configuration or install additional hard drives in the future.

Well Grouped

LVM adds an abstraction layer between mass storage and its partitions and filesystems. You can thus combine several mass storage devices or physical volumes with LVM in a volume group and then address them as a unit. If only one mass storage device is available, you can set up one or more volume groups on it and then create logical volumes that also contain the respective filesystems. The smallest units of a logical volume system are the physical extents (4MB by default), which are comparable to sectors in classical partitioning schemes.

If you want to add additional mass storage, you retroactively grow the volume groups. The individual logical volumes can grow or shrink as needed, without the need to reconfigure the storage media or recreate the filesystem. It should be noted that the filesystem does not always permit growing and shrinking: Some modern candidates (e.g., XFS [1], JFS [2]) can only grow volumes in the mounted state, not shrink them.

Linux LVM also lets you rearrange data on the fly, which allows hot-swappable configurations, for example, in cluster environments. You can optimize the configuration of your mass storage for performance or safety through different distribution and mirror functions (see the "LVM and RAID" box). LVM Snapshots let you save certain defined conditions and protect yourself against failure.

Disadvantages of LVM

Depending on the configuration, the disadvantages of using LVM include an increased risk of losing data if the system distributes it to multiple devices and one of the disks fails. You should always store data redundantly or make backups in good time. The initial setup is more difficult than with conventional partitions, as well.

LVM on Linux

Linux does not need any special conditions to use LVM, because the operating system supports the functionality (since kernel 2.4). When you install the operating system on single-user systems or small servers, the wizards in all major distributions offer you the option of setting up LVM along with the mass storage. The implementation sometimes causes confusion, though.

Depending on the distribution, the installer creates different volume groups and physical volumes for data partitions and the swap file. Typically, it also creates a /boot partition that occupies several hundred megabytes on the device. This also happens if you select an unsupported filesystem such as Btrfs from the GRUB boot manager (Figure 1).

Figure 1: If you want to create a new system with LVM, the installer independently sets up a few volume groups.

You can set up other existing physical mass storage later, if necessary; this also applies to the configuration of the existing storage media. However, the command sequences to be entered at the prompt have many parameters and require training. Graphical tools mainly help LVM newcomers with the software.

In this article, then, I examine a few of these tools in depth, paying particular attention to their practicality in small environments. I also place emphasis on simple operation of the software to clarify whether it can be used without extensive background knowledge.