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Changing the VVR tunables

VVR provides you with a number of tunable parameters that can be tuned to a specific value, according to your requirements. For a detailed explanation of the VVR tunables, see the Veritas Volume Replicator Planning and Tuning Guide.

The following table provides you with a quick reference to the tunables that can be modified using the vxtune utility. All tunables can be modified using the system-specific method.

  Note   The volpagemod_max_memsz is a VxVM tunable that is used to specify the amount of memory, measured in kilobytes, required for caching FastResync and cache object metadata. For more information on using this tunable, refer to "Performance Tuning and Monitoring" in the Veritas Volume Manager Administrator's Guide.

The tunables can be tuned either using the vxtune utility or by using the system-specific interface. Some of the tunables that can be tuned using only the system-specific method, while the others such as the memory tunables can be tuned using both the methods. The advantage of using the vxtune utility to tune the parameters is that you need not reboot the system after modifying the tunable value. This is especially useful if you want to experiment with different values to arrive at an optimum value to suit your requirements. However, the changes to the tunables using the vxtune utility are non-persistent. To make the changes persistent you still must use the system-specific method. However, you must reboot the system for the changes to take effect.

The current values for the tunables are defined in the /etc/vx/vxtunables file after you have used the vxtune utility for the first time.

Points to note when changing the value of the tunables

• When decreasing the value of the vol_rvio_maxpool_sz tunable, all the RVGs on the host must be stopped.
• When decreasing the size of the tunables vol_max_rdback_sz and vol_max_nmpool_sz, and vol_max_wrspool_sz, pause the RLINKs.
• The vol_min_lowmem_sz tunable is auto-tunable; depending on the incoming writes VVR increases or decreases the tunable value.

In a shared disk group environment you may choose to set only those tunables that are required on each host. However, we recommend that you configure the tunables appropriately even if the tunables are currently not being used. This is because if the logowner changes, then tunables on the new logowner will be used. The following list of tunables are required to be set only on the logowner and not the other hosts:

• vol_max_rdback_sz
• vol_max_nmpool_sz
• vol_max_wrspool_sz
• vol_dcm_replay_size
• vol_nm_hb_timeout
• vol_vvr_use_nat

The tunable changes that are done using the vxtune command affects only the tunable values on the host on which it is run. Therefore, in a shared disk group environment, you must run the command separately on each host for which you want to change the tunable values.