HP-UX System Administrator's Guide: Logical Volume Management (762803-001, March 2014)

Mirroring Simultaneous replication of data, ensuring a greater degree of data availability. LVM can map
identical logical volumes to multiple LVM disks, thus providing the means to recover easily from
the loss of one copy (or multiple copies in the case of multi-way mirroring) of data. Mirroring can
provide faster access to data for applications using more data reads than writes. Mirroring
requires the MirrorDisk/UX product.
Persistent Device
Special Files (DSFs)
A Device Special File conforming to the naming model introduced in HP-UX 11i v3 to support
agile addressing. The Device Sepcial File name contains an instance number, such as
/dev/disk/disk#, and the minor number has no hardware path information. Persistent DSFs
provide a persistent, path-independent representation of a device bound to the device's LUN
hardware path and World Wid Identifier (WWID). The persistent DSFs represent the LUN itself
(rather than a specific path to a LUN such as legacy DSFs), so a single/persistent DSF is created
for a LUN regardless of the number of underlying paths to the LUN.
Physical Extents Fixed-size addressable areas of space on an LVM disk. They are the basic allocation units for a
physical volume. Physical extents map to areas on logical volumes called logical extents.
Physical Volume A disk that has been initialized by LVM for inclusion in a volume group; also called an LVM disk.
As with standard disks, an LVM disk (physical volume) is accessed via a raw device file (for
example, /dev/rdisk/disk3). Use the HP SMH or the pvcreate command to initialize a
disk as a physical volume.
Physical Volume
Group
A subset of physical volumes within a volume group, each with a separate I/O channel or interface
adapter to achieve higher availability of mirrored data.
Quorum The requirement that a certain number of LVM disks be present in order to change or activate a
volume group. To activate a volume group, quorum requires the number of available LVM disks
to be more than half the number of configured LVM disks that were present when the volume
group was last active. To make a configuration change, the quorum requirement is at least half.
If there is no quorum, LVM prevents the operation. Quorum is checked both during configuration
changes (for example, when creating a logical volume) and at state changes (for example, if a
disk fails). Quorum ensures the consistency and integrity of the volume groups. The vgchange
command with the -q n option can be used to override quorum check, but this should be used
with caution.
Snapshot Logical
Volume
A point-in-time image of a logical volume, used to create another copy of the logical volume
without taking up as much of the physical space as the size of the logical volume.
Striping Disk striping distributes logically contiguous data blocks (for example, chunks of the same file)
across multiple disks, which speeds up I/O throughput for large files when they are read and
written sequentially (but not necessarily when access is random).
Synchronization The process of updating stale (non-current) copies of mirrored logical extents by copying data
from a fresh (current) copy of the logical volume. Synchronization keeps mirrored logical volumes
consistent by ensuring that all copies contain the same data.
Unit of Information The units of information used are as follows:
• KB is a kilobyte unit of information equal to 2
10
or 1024 bytes.
• MB is a megabyte unit of information equal to 2
20
or 1,048,576 bytes.
• GB is a gigabyte unit of information equal to 2
30
or 1,073,741,824 bytes.
• TB is a terabyte unit of information equal to 2
40
or 1,099,511,627,776 bytes.
• PB is a petabyte unit of information equal to 2
50
or 1,125,899,906,842,624 bytes.
Unshare Unit The smallest unit at which data is unshared between a logical volume and its snapshot when a
write occurs onto the snapshot or its successor.
Volume Group A collection of one or more LVM disks from which disk space may be allocated to individual
logical volumes. A disk can belong to only one volume group. A volume group is accessed
through the group file (for example, /dev/vg01/group) in that volume group's directory. Use
HP SMH or the vgcreate command to create a volume group.
194 Glossary