The concurrency capability of an HBA indicates the maximum number of I/Os that each LUN can transmit at a time. If this is insufficiently low for a high concurrency scenario, it typically leads to poor performance.
Windows Hosts
In Windows, the concurrency capability of most HBAs is 128. However, in certain Windows versions, this value may be relatively small. For example, in Windows Server 2012 R2, the corresponding value of an Emulex HBA is 32. Insufficient concurrency capability leads to the issue of the host pressure not being fully transferred to the storage side. If the difference between the latency on both sides is large, you can use the management software provided by the HBA vendor to query the concurrency capability of the HBA and set it to an appropriate value as necessary.
Linux Hosts
In Linux, the queue parameter settings of an HBA vary depending on its type and driver. For details, see the specifications provided by the HBA vendor. For example, the QLogic 8 Gbit/s dual-port Fibre Channel HBA allows the maximum queue depth of each LUN to be 32. If the latency difference between the host side and the storage side is large, run iostat to check whether the concurrency bottleneck is reached.
In the above figure, the highlighted box shows the entries under avgqu-sz, which represent the average queue depth of the block device corresponding to a LUN. If the value of avgqu-sz is 10 or larger for a long time, it is possible that I/Os are accumulated on the block device layer of the host due to the concurrency limitation. The host pressure is not transferred to the storage side. In this case, you can increase the concurrency capability of the HBA.
