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. 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 3.1 Administrator's
Guide for more information. VxVM volumes can be one
of four types: mirrored, RAID-5, striped, or concatenated. |
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. |
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 |
/etc/lvmtab is an LVM file that contains information about volume
groups that are accessible by a system. |
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