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

Administering a System: Managing Disks and Files
Managing Disks
Chapter 6562
As can be seen in Figure 6-2 on page 561, the contents of the first
logical volume are contained on all three physical volumes in the
volume group. Since the second logical volume is mirrored, each
logical extent is mapped to more than one physical extent. In this
case, there are two physical extents containing the data, each on
both the second and third disks within the volume group.
• By default, LVM assigns physical extents to logical volumes by
selecting available physical extents from disks in the order you
added physical volumes to the volume group. As a system
administrator, you can bypass this default assignment and control
which disks are used by a logical volume (see “Extending a Logical
Volume to a Specific Disk” on page 577).
• If a logical volume is to be used for root, boot, primary swap, or
dump, the physical extents must be contiguous. This means that
the physical extents must be allocated with no gaps on a single
physical volume. On non-root disks, physical extents that correspond
to contiguous logical extents within a logical volume can be
noncontiguous on a physical volume or reside on entirely different
disks. As a result, it becomes possible for a file system created within
one logical volume to reside on more than one disk.
Planning for the Use of Logical Volumes
Using logical volumes requires some planning. Some of the issues you
should consider for planning purposes are listed below and discussed in
the remainder of this section. You should consider these issues before
setting up or modifying logical volumes on your system.
• For what purpose will you use a logical volume? For a file system, for
swap space, or for raw data storage? You can also use a logical
volume for booting the system or as a dump area; see “Creating Root
Volume Group and Root and Boot Logical Volumes” on page 578 for
details.
• How big should you make a logical volume?
• Is I/O performance very important to you? If so, you need to consider
your disk interface types and models.
• Does your data require high availability? If so, see information on
mirroring. Also see the information under “Increasing Availability
with Alternate Links” on page 568.