Nonhomogeneous queue depth calculation in iSCSI hosts

After you understand certain criteria for nonhomogeneous queues, you can calculate their queue depth in iSCSI hosts.

Nonhomogeneous queues meet one of the following criteria:

Set the queue depth for each volume on the servers by using the following parameters:

For each volume, consider each server to which that volume has a mapping in an I/O group. This mapping results in a set of server/volume pairs. If the sum of the server and volume queue depth for all of the pairs is less than 4000, the server does not experience problems due to a full queue.

Example

Consider a situation with 20 host servers and 2 I/O groups, iogrp0 and iogrp1. In iogrp0, 20 volumes are mapped to 20 host servers. In iogrp1, 40 volumes are mapped to the same 20 host servers. Using the formula that is described in Homogeneous queue depth calculation in iSCSI hosts, the following values are derived:

  • For iogrp0:
    • v = 20
    • c = 20
    • q = (4000/20 * 20) * 0.75 = 7 (considering the lower boundary limit)

    The total server-to-volume mapping is 400 (20 times 20), and queue depth for each volume is seven. Therefore, the sum of the queue depths for all mappings is 2800 (400 times seven). Because this amount is less than 4000, problems of full queues do not occur.

  • For iogrp1:
    • v = 40
    • c = 20
    • q = (4000/40 * 20)) * 0.75 = 3 (considering the lower boundary limit)

    The total server-to-volume mapping is 800 (40 times 20), and queue depth for each volume is three. Therefore, the sum of the queue depths for all mappings is 2400 (800 times 3). Because this amount is less than 4000, problems of full queues do not occur.

Parent topic: Queue depth in iSCSI hosts