Homogeneous queue depth calculation in iSCSI hosts

You must set the homogeneous queue depth to a calculated value in iSCSI hosts.

Set the queue depth for each volume on the servers by using the calculation

• q = (n/(v * c)) * 0.75

where

• n = the allowed command pool per I/O group; this value is fixed at 4000
• v = number of volumes per server for the particular I/O group under consideration
• c = number of servers in an I/O group

Therefore, q = (4000/(v * c)) * 0.75.

Example

Consider a system with 20 host servers (c = 20), each with 20 volumes mapped (v = 20). For this configuration, the value of the queue depth is q = (4000/(20 * 20)) * 0.75 = 7.5 (or the lower boundary limit of 7)

Use the minimum value of 2 if the value of derived queue depth exceeds the maximum allowed value on a particular host operating system.

Some common host server configurations are summarized in Table 1, considering 20 volumes mapped to each host server.