Managing Systems and Workgroups: A Guide for HP-UX System Administrators

Administering a System: Managing Disks and Files

Managing Disks

Chapter 6560

incrementing the address by one for each unit. Physical extent size is

conﬁgurable at the time you form a volume group and applies to all

disks in the volume group. By default, each physical extent has a size

of 4 megabytes (MB). This value can be changed when you create the

volume group to a value between 1MB and 256MB.

• The basic allocation unit for a logical volume is called a logical

extent. A logical extent is mapped to a physical extent; thus, if the

physical extent size is 4MB, so will be the logical extent size. The size

of a logical volume is determined by the number of logical extents

conﬁgured.

• When LVM allocates disk space to a logical volume, it automatically

creates a mapping of the physical extents to logical extents. Logical

extents are also allocated sequentially, starting at zero, for each

logical volume. Therefore, regardless of where the actual physical

data resides for a logical volume within a volume group, LVM will

use this mapping to access the data. Commands are provided for you

to examine this mapping; see pvdisplay (1M) and lvdisplay (1M).

• Except for mirrored or striped logical volumes, each logical extent is

mapped to one physical extent. For mirrored logical volumes, either

two or three physical extents are mapped for each logical extent

depending upon whether you are using single or double mirroring.

For example, if one mirror copy exists, then each logical extent maps

to two physical extents, one extent for the original and one for the

mirror copy. See “Managing Mirrored File Systems” on page 627 for

more information on mirroring. For information on striped logical

volumes, see “Setting Up Disk Striping” on page 589. See also the

book Disk and File Management Tasks on HP-UX.