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Logical Volume ManagementLogical volume management (LVM) is a hard disk drive partitioning scheme that is designed to be more flexible than normal physical partitioning. In particular, logical volume management software allows for changes in the size of individual volumes without a hard reboot of the computer, and in some cases while the filesystem on the volume is being actively used. A few operating systems have incorporated logical volume management in their design, such as Linux, HP-UX and AIX. Linux kernel 2.4 contains LVM version 1, and kernel 2.6 contains LVM version 2. LVM implementations differ but some basic concepts exist across most versions. Storage devices, commonly hard disks (also RAID devices or SAN LUNs, etc.), are called Physical Volumes or PVs. For LVM to use a PV, the PV is split into Physical Extents or PEs. PEs are small chunks (HP-UX defaults to 4 megabyte PEs) of a PV that will be masked as Logical Extents or LEs to the user of LVM. Once split into these small parts, the PV is added to a Volume Group or VG. A volume group must be used even if there is only one disk in the volume group. Once the PV has been added to the volume group, LEs may be grouped together to form Logical Volumes or LVs. LVs are analagous to traditional disk partitions. Once an LV is created, a filesystem may be put on it (or not) which can then be mounted by the operating system. LEs may be added (and sometimes removed) from an LV to change its size. Changing the size of the LV doesn't change the size of the filesystem, however; so a filesystem that supports resizing is recommended, but not necessary, for use in an LVM environment. Additional PVs may be added to a volume group to create logical volumes that are larger than the physical volumes that contain them or for data redundancy (mirroring). PVs may also be organized into Physical Volume Groups or PVGs. The use of PVGs allow for software mirroring of logical extents, ensuring that a mirror LE isn't on the same PV as the LE being mirrored. External links Linux
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