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Tuning guidelines for large systems

On smaller systems (with fewer than a hundred disk drives), tuning is unnecessary and VxVM is capable of adopting reasonable defaults for all configuration parameters. On larger systems, configurations can require additional control over the tuning of these parameters, both for capacity and performance reasons.

Generally, only a few significant decisions must be made when setting up VxVM on a large system. One is to decide on the size of the disk groups and the number of configuration copies to maintain for each disk group. Another is to choose the size of the private region for all the disks in a disk group.

Larger disk groups have the advantage of providing a larger free-space pool for the vxassist command to select from, and also allow for the creation of larger volumes. Smaller disk groups do not require as large a configuration database and so can exist with smaller private regions. Very large disk groups can eventually exhaust the private region size in the disk group with the result that no more configuration objects can be added to that disk group. At that point, the configuration either has to be split into multiple disk groups, or the private regions have to be enlarged. This involves re-initializing each disk in the disk group (and can involve reconfiguring everything and restoring from backup).

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 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