VERITAS Volume Manager 4.1 Administrator's Guide

Performance Monitoring and Tuning
Tuning VxVM
Chapter 12 407
A general recommendation for users of disk array subsystems is to create
a single disk group for each array so the disk group can be physically
moved as a unit between systems.
Number of Configuration Copies for a Disk Group
Selection of the number of configuration copies for a disk group is based
on a trade-off between redundancy and performance. As a general rule,
reducing the number configuration copies in a disk group speeds up
initial access of the disk group, initial startup of the vxconfigd daemon,
and transactions performed within the disk group. However, reducing
the number of configuration copies also increases the risk of complete
loss of the configuration database, which results in the loss of all objects
in the database and of all data in the disk group.
The default policy for configuration copies in the disk group is to allocate
a configuration copy for each controller identified in the disk group, or for
each target that contains multiple addressable disks. This provides a
sufficient degree of redundancy, but can lead to a large number of
configuration copies under some circumstances. If this is the case, we
recommended that you limit the number of configuration copies to a
minimum of 4. Distribute the copies across separate controllers or
targets to enhance the effectiveness of this redundancy.
To set the number of configuration copies for a new disk group, use the
nconfig operand with the vxdg init command (see the vxdg (1M)
manual page for details).
You can also change the number of copies for an existing group by using
the vxedit set command (see the vxedit (1M) manual page). For
example, to configure five configuration copies for the disk group, bigdg,
use the following command:
# vxedit set nconfig=5 bigdg
Changing Values of Tunables
Tunables are modified by adding a line to /stand/system file. Changed
tunables take effect only after relinking the kernel and booting the
system from the new kernel.
Modify tunables in /stand/system using the following format:
tunable value