Homogeneous queue depth calculation in iSCSI hosts
You must set the homogeneous queue depth to a calculated value in iSCSI hosts.
The homogeneous
queues must meet the following criteria:
- The queued commands must be shared among all paths rather than providing servers with additional resources.
- The volumes must be distributed evenly among the I/O groups in the clustered system.
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.
| Number of iSCSI hosts per I/O group | <20 | 20-40 | 64 |
|---|---|---|---|
| Recommended queue depth | Seven (or the maximum number that is allowed by the host OS, whichever is smaller) | Three to four (or the max number that is allowed by the host OS, whichever is smaller) | Two |