Kalum Umesh

In the olden days computer disks were small compared to the data set sizes that were needed. A solution is to make several physical disks appear virtually (or logically) as a single much larger disk. A large filesystem could then be created on that virtual disk.

Later hard disk sizes grew large enough that it made sense to do the opposite: make one physical disk appear as several virtual disks. Each virtual disk holds a filesystem independently of the others. Today such virtual disks are called disk "partitions".

Now we have come full circle. Large data warehouse applications require very large filesystems to hold the database data files. To support this sort of application the old idea of combining several disks into one has been resurrected. Novell Netware supported this feature since the 1990s. The physical disk (or selected disk partitions) are formatted to be "physical volume segments". All added physical volume segments become part of a single large virtual disk. The administrator can then create logical "volumes", that is, a filesystem. The exciting part is that if some volume is low on space, you can extend the virtual disk by adding another physical volume segment to it, and then increase the (logical) volume's size. This operation is fast and doesn't disturb the existing data or other partitions (or volumes)!

The modern Unix (and Linux) version of this idea is called "Logical Volume Management" (or "LVM"). LVM allows the administrator to

• use and allocate disk space more efficiently and flexibly
• move logical volumes between different physical devices
• have very large logical volumes span a number of physical devices
• take snapshots of whole filesystems easily, allowing on-line backup of those filesystems
• replace on-line drives without interrupting services