Managing Systems and Workgroups: A Guide for HP-UX System Administrators
Administering a System: Managing Disks and Files
Managing Disks
Chapter 6566
Typically, you specify the size of a logical volume in megabytes. However,
a logical volume’s size must be a multiple of the extent size used in the
volume group. By default, the size of each logical extent is 4 MB.
So, for example, if a database partition requires 33MB and the default
logical extent size is 4 MB, LVM will create a logical volume that is 36MB
(or 9 logical extents).
The maximum supported size for a raw data device is 4 GB.
If you plan to use logical volumes heavily for raw data storage (such as
for setting up database partitions), you should consider how the logical
volumes are distributed over your disks.
By default, LVM will assign disk space for a logical volume from one
disk, use up the space on this disk entirely, and then assign space from
each successive disk in the same manner. LVM uses the disks in the
order in which they were added to the volume group. This means that a
logical volume’s data may not turn out to be evenly distributed over all
the disks within your volume group.
As a result, when I/O access to the logical volumes occurs, one or more
disks within the volume group may be heavily used, while the others
may be lightly used, or not even used at all. This arrangement does not
provide optimum I/O performance.
As a better alternative, you can set up your logical volume on specific
disks in an interleaved manner, thus balancing the I/O access and
optimizing performance. (See “Extending a Logical Volume to a Specific
Disk” on page 577.)
Because there are no HP-UX commands that will identify that the
contents of a logical volume are being used for raw data, it is a good idea
to name the logical volumes you create for raw data with easily
recognizable names. In this way, you can recognize the contents of such a
logical volume. See “Naming Logical Volumes” on page 570 for more
information.