VxVM and LVM---Conceptual Comparison

LVM

VxVM

Both LVM and VxVM enable online disk storage management. They both build virtual devices, called volumes, on physical disks. Volumes are not limited by the underlying physical disks, and can include other virtual objects such as mirrors. Volumes are accessed through the HP-UX file system, a database, or other applications in the same manner as physical disks would be accessed.

Physical Volume

VxVM Disk

An LVM physical volume and a VxVM disk are conceptually the same. A physical disk is the basic storage device (media) where the data is ultimately stored. You can access the data on a physical disk by using a device name (devname) to locate the disk.

In LVM, a disk that has been initialized by LVM becomes known as a physical volume.

A VxVM disk is one that is placed under the Volume Manager control and is added to a disk group.

VxVM can place a disk under its control without adding it to a disk group. The VxVM Storage Administrator shows these disks as "free space pool".

Logical Volume

Volume

An LVM logical volume and a VxVM volume are conceptually the same. Both are virtual disk devices that appear to applications, databases, and file systems like physical disk devices, but do not have the physical limitations of physical disk devices. Due to its virtual nature, a volume (LVM or VxVM) is not restricted to a particular disk or a specific area of a disk.

An LVM volume is composed of fixed length extents. LVM volumes can be mirrored or striped, but mirrored-stripe and striped-mirror layouts are not supported.

VxVM volumes consist of one or more plexes/mirrors holding a copy of the data in the volume which in turn are made up of subdisks with arbitrary length. The configuration of a volume can be changed by using the VxVM user interfaces. See the VERITAS Volume Manager Administrator's Guide for more information. VxVM volumes can be concatenated, mirrored, striped, RAID-5 or combinations such as mirrored-stripe, striped-mirror, and concatenated-mirror.

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.

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.

Export

Deport

In LVM, exporting removes volume group information from /etc/lvmtab. The volume group must have already been deactivated.

Similarly in VxVM, deport makes a disk group inaccessible by the system.

Import

Import

In LVM, import adds a volume group to the system and the volume group information to /etc/lvmtab but does not make the volumes accessible. The volume group must be activated by the vgchange -a y command in order to make volumes accessible.

In VxVM, import imports a disk group and makes the diskgroup accessible by the system.

Bad Block Pool

No similar term

In LVM, the bad block pool provides for the transparent detection of bad disk sectors, and the relocation of data from bad to good disk sectors.

The bad block reallocation feature does not exist in VxVM because the vectoring of bad blocks is now done by most hardware.

/etc/lvmtab

No similar term

The /etc/lvmtab file contains information about volume groups that are accessible by a system.