Homogeneous queue depth calculation in iSCSI hosts

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

Queued commands in the homogeneous queues must be shared among all paths rather than providing servers with additional resources.

Set the queue depth for each volume on the servers 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 having 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.