VERITAS Volume Manager 3.5 Hardware Notes (June 2002)

Hardware Notes 9
BETA 19 July 2002 Controlling Path Switching of I/O over DMP Paths
Controlling Path Switching of I/O over DMP Paths
The dmp_pathswitch_blks_shift tunable may be used to control the number of
contiguous I/O blocks (expressed as an integer power of 2) that are sent along a DMP
path to an Active/Active array before switching to the next available path.
The default value of this parameter is set to 10 so that 1024 blocks (2^10 * 1024 bytes or 1
MB) of contiguous I/O are sent over a DMP path before switching. For intelligent disk
arrays with internal data caches, better throughput may be obtained by increasing the
value of this tunable. For example, for the HDS 9960 A/A array, the optimal value is
between 14 and 16 for an I/O activity pattern that consists mostly of sequential reads or
writes.
The DMP path that is used for a block is calculated as follows:
path = (block# >> dmp_pathswitch_blks_shift) % number of paths
To changethe valueof thedmp_pathswitch_blks_shift tunable, create anentryfor it
in the /kernel/drv/vxdmp.conf file, such as the following:
dmp_pathswitch_blks_shift=15
This example would send 32MB of contiguous I/O over a DMP path before switching.
After saving your changes to the configuration file, reboot the system to have the change
take effect. You can use the prtconf -vP command to display the current values of all
tunables.
For more information see the “Performance Monitoring and Tuning” chapter in the
VERITAS Volume Manager Administrator’s Guide.
DMP and Hitachi Data Systems Fiber Disk Array Subsystem
Support
Models 5800E and 9200
DMP supports the Hitachi Data Systems 5800E/9200 Disk Array Subsystems in both the
Active/Passive and Active/Active modes. Depending on the options specified in the
array settings, DMP claims the array as appropriate. The recommended mode for the
array is Active/Passive.
For detailed array specific information, please contact your Hitachi Data Systems
Technical Support Representative.