Physical Versus Logical Extents - HP -UX 11i Administrator's Manual

Figure 1 Disk Space Partitioned Into Logical Volumes

Physical versus Logical Extents

When LVM allocates disk space to a logical volume, it automatically creates a mapping of the
logical extents to physical extents. This mapping depends on the policy chosen when creating the
logical volume. Logical extents are allocated sequentially, starting at zero, for each logical volume.
LVM uses this mapping to access the data, regardless of where it physically resides. Commands
are provided for you to examine this mapping; see pvdisplay(1M) and lvdisplay(1M).
Except for mirrored, striped, or striped-mirrored logical volumes, each logical extent is mapped to
one physical extent. For mirrored logical volumes, each logical extent is mapped to multiple physical
extents, depending on the number of mirror copies. 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. For more information on mirroring, see
(page 24). For information on striped logical volumes, see
Striping" (page
Figure 2 shows an example of several types of mapping available between physical extents and
logical extents within a volume group.
10 Introduction
31). Also refer to the book Disk and File Management Tasks on HP-UX.
"Increasing Data Availability Through Mirroring"
"Increasing Performance Through Disk