VERITAS Volume Manager 5.0 Migration Guide (September 2006)

11VxVM and LVM
VxVM and LVM—conceptual comparison
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 contiguous 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.
Table 1-1 A conceptual comparison of LVM and VxVM
LVM term VxVM term