Veritas Volume Manager 5.0 Administrator's Guide (September 2006)

461Performance monitoring and tuning
Tuning VxVM
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 maximum 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 the values of tunables
Tunables are modified by using SAM or the kctune utility. Changed tunables take effect
only after relinking the kernel and booting the system from the new kernel.
The values of system tunables can be examined by selecting Kernel Configuration
> Configuration Parameters in SAM.
DMP tunables may be set by using the vxdmpadm command as shown here:
# vxdmpadm settune dmp_tunable=value
The values of these tunables can be displayed by using this command:
# vxdmpadm gettune [dmp_tunable]
The vxdmpadm command also allows you to configure how DMP responds to I/O errors at
the level of the paths to individual arrays. See “Administering DMP using vxdmpadm” on
page 133 for details.