Logical Volume Manager (Linux)

LVM is a logical volume manager for the allLinux kernel. It was originally written in 1998 by Heinz Mauelshagen, who based its design on that of the LVM in HP-UX.

What it manages is disk drives and similar mass-storage devices, in particular large ones. By "volume" is meant a disk drive, or part of one.

The installers for the Fedora, MontaVista Linux, openSUSE, SLES, SLED, Debian GNU/Linux, and Ubuntu distributions are LVM-aware and can install a bootable system with a root filesystem on a logical volume.

Features

The LVM can:

• Resize volume groups online by absorbing new physical volumes (PV) or ejecting existing ones.
• Resize logical volumes (LV) online by concatenating extents onto them or truncating extents from them.
• Create read-only snapshots of logical volumes (LVM1).
• Create read-write snapshots of logical volumes (LVM2).
• Stripe whole or parts of logical volumes across multiple PVs, in a fashion similar to RAID0.
• Mirror whole or parts of logical volumes, in a fashion similar to RAID1.
• Move online logical volumes between PVs.
• Split or merge volume groups in situ (as long as no logical volumes span the split). This can be useful when migrating whole logical volumes to or from offline storage.

The LVM will also work in a shared-storage cluster (where disks holding the PVs are shared between multiple host computers), but requires an additional daemon to propagate state changes between cluster nodes.

LVM does not:

• Provide parity-based redundancy across LVs, as with RAID4, RAID5 or RAID6. This functionality is instead provided by Linux metadisks, which can be used as LVM physical volumes.

Implementation

LVM keeps a metadata header at the start of every PV, each of which is uniquely identified by a UUID. Each PV's header is a complete copy of the entire volume group's layout, including the UUIDs of all other PV, the UUIDs of all logical volumes and an allocation map of PEs to LEs. This simplifies data recovery in the event of PV loss.

In the 2.6-series Linux kernels, the LVM is implemented in terms of the device mapper, a simple block-level scheme for creating virtual block devices and mapping their contents onto other block devices. This minimizes the amount of relatively hard-to-debug kernel code needed to implement the LVM. It also allows its I/O redirection services to be shared with other volume managers (such as EVMS). Any LVM-specific code is pushed out into its user-space tools, which merely manipulate these mappings and reconstruct their state from on-disk metadata upon each invocation.

To bring a volume group online, the "vgchange" tool:

1. Searches for PVs in all available block devices.
2. Parses the metadata header in each PV found.
3. Computes the layouts of all visible volume groups.
4. Loops over each logical volume in the volume group to be brought online and:
   1. Checks if the logical volume to be brought online has all its PVs visible.
   2. Creates a new, empty device mapping.
   3. Maps it (with the "linear" target) onto the data areas of the PVs the logical volume belongs to.

To move an online logical volume between PVs, the "pvmove" tool:

1. Creates a new, empty device mapping for the destination.
2. Applies the "mirror" target to the original and destination maps. The kernel will start the mirror in "degraded" mode and begin copying data from the original to the destination to bring it into sync.
3. Replaces the original mapping with the destination when the mirror comes into sync, then destroys the original.

These device mapper operations take place transparently, without applications or filesystems being aware that their underlying storage is moving.

Caveats

The current implementation does not support write barriers. This means that the guarantee against filesystem corruption offered by journaled file systems like ext3 and XFS is negated under some circumstances.

References

• Lewis, AJ (2006-11-27). "LVM HOWTO". Linux Documentation Project. Retrieved 2008-03-04..
• US patent 5129088, Auslander, et al, "Data Processing Method to Create Virtual Disks from Non-Contiguous Groups of Logically Contiguous Addressable Blocks of Direct Access Storage Device", issued 1992-7-7  (fundamental patent).

External links

• LVM2 Resource Page