VERITAS Volume Manager 3.5 Migration Guide (September 2004)

Chapter 1
VxVM and LVM
VxVM and LVM—Conceptual Comparison
5
Volume Group Disk Group
LVM volume groups are conceptually similar to VxVM disk groups.
An LVM volume group is the collective identity of a set of physical volumes, which provide disk storage for
the logical volumes.
A VxVM disk group is a collection of VxVM disks that share a common configuration. A configuration is a
set of records with detailed information about related VxVM objects, their attributes, and their
associations.
In addition, both LVM and VxVM have the following characteristics:
Volumes can be mapped to multiple VxVM disks or LVM physical volumes.
VxVM disks must reside in only one disk group, and LVM physical volumes must reside in one volume
group.
Physical Extent Subdisk
User data is contained in physical extents in LVM and subdisks in VxVM.
The LVM physical extents are of a fixed length. LVM allocates space in terms of physical extents which is a
set of physical disk blocks on a physical volume. The extent size for all physical volumes within a volume
group must be the same, and is usually 4 MB.
VxVM allocates disk space in term of subdisks which is a set of physical disk blocks representing a specific
portion of a VxVM disk and is of arbitrary size.
LVM metadata Private Region
LVM metadata and the Private Region are similar conceptually.
In LVM, metadata is stored in a reserved area in the disk.
In VxVM, the private region of a disk contains various on-disk structures that are used by the Volume
Manager for various internal purposes. Private regions can also contain copies of a disk group’s
configuration, and copies of the disk group’s kernel log.
Unused Physical Extent Free Space
LVM contains unused physical extents that are not part of a logical volume, but are part of the volume
group.
Similarly, free space is an area of a disk under VxVM that is not allocated to any subdisk or reserved for
use by any other Volume Manager object.
Mirrors Mirrors (Plexes)
Both LVM and VxVM support mirrors. Mirrors can be used to store multiple copies of a volume’s data on
separate disks.
In LVM, you can create mirrors using the MirrorDisk/UX product. Mirrors allow duplicate copies of the
extents to be kept on separate physical volumes. MirrorDisk/UX supports up to 3 copies of the data.
A VxVM mirror consists of plexes. Each plex is a copy of the volume. A plex consists of one or more
subdisks located on one or more disks. VxVM volumes can have up to 32 mirrors (where each plex is a copy
of data). Mirroring features are available with an additional license.
Table 1-1 A Conceptual comparison of LVM and VxVM (Continued)
LVM Term VxVM Term