7.6.2 HBA parameters for performance tuning Transaction-based settings The queue_depth hdisk attribute The num_cmd_elem attribute  Throughput-based settings You can use the example settings in this section to start your configuration in the specific  workload environment. These settings are a guideline, and are not guaranteed to be the  answer to all configurations. Always try to set up a test of your data with your configuration to  see whether further tuning can help. For best results, it helps to have knowledge about your  specific data I/O pattern. The settings in the following sections can affect performance on an AIX host. These sections  examine these settings in relation to how they affect the two workload types. The host attachment script sets the default values of attributes for IBM Spectrum Virtualize  hdisks:  devices.fcp.disk.IBM.rte  or  devices.fcp.disk.IBM.mpio.rte . You can modify  these values as a starting point. In addition, you can use several HBA parameters to set  higher performance or large numbers of hdisk configurations.  You can change all attribute values that are changeable by using the  chdev  command for AIX.  AIX settings that can directly affect transaction performance are the  queue_depth  hdisk  attribute and  num_cmd_elem  attribute in the HBA attributes. For the logical drive (which is known as the hdisk in AIX), the setting is the attribute  queue_depth , as shown in the following example: # chdev -l hdisk X  -a queue_depth= Y  -P In this example,  X  is the hdisk number, and  Y  is the value to which you are setting X  for  queue_depth . For a high-volume transaction workload of small random transfers, try a  queue_depth  value of  25  or more. For large sequential workloads, performance is better with shallow queue depths,  such as a value of  4 . For the HBA settings, the  num_cmd_elem  attribute for the fcs device represents the number of  commands that can be queued to the adapter, as shown in the following example: chdev -l fcs X  -a num_cmd_elems=2048 -P The default value is 200, but the following maximum values can be used, depending on your  HBA vendor. The AIX settings that can directly affect throughput performance with large I/O block size are  the  lg_term_dma  and  max_xfer_size  parameters for the fcs device. In the throughput-based environment, you might want to decrease the queue-depth setting to  a smaller value than the default from the host attach. In a mixed application environment, you  do not want to lower the  num_cmd_elem  setting because other logical drives might need this  higher value to perform. In a purely high throughput workload, this value has no effect. Start values:  For high throughput sequential I/O environments, use the start values  lg_term_dma  =  0x400000  or  0x800000  (depending on the adapter type) and  max_xfr_size  =  0x200000 . Chapter 7. Hosts  259