Queue depth in Fibre Channel hosts

In Fibre Channel networks, the queue depth is the number of I/O operations (SCSI commands) that can be run in parallel on a device.

Usually a higher queue depth can lead to better performance. However, when too many concurrent I/Os are sent to a storage device, the device responds with an I/O failure message of queue full. This message is intended to cause the host to try the I/O again a short time later. However, not all operating systems correctly handle the queue-full failure, which can cause unnecessary I/O failures or delays to applications.

To avoid these delays or failures, configure your hosts so that they send only a finite number of I/Os to the storage system to avoid exhausting the resources of the storage system.

In configurations with multiple hosts, configure each host with a similar queue depth to maintain fairness between the hosts.

For most hosts, set the HBA queue depth to 32.
For hosts that are significantly busier or where not many hosts are configured on the storage system, set the HBA queue depth to 128.

Ensure that the queue depth total for all hosts that are connected to a single physical Fibre Channel port on the storage system does not exceed 2048.

If host I/O queue full failures regularly occur, consider reducing the queue depths or distributing the host zoning across more physical Fibre Channel ports on the storage system.