Once again, I am resurrecting an old post from my old blog…!

Yesterday, I helped fix an issue with an iSCSI where we were seeing output drops on all iSCSI interfaces. I spent a few hours yesterday trying to figure out why we were seeing these drops – initally I thought it might have been a cabling issue, but after a quick Google I soon eliminated that as the issue.

It would seem that the drops were occuring because of QoS being enabled on the switch. Because iSCSI is very “chatty”, the switch was queuing the packets and then dropping them as and when the queue was becoming to large. We then disabled the QoS on the switch, and lo and behold there were no output drops on any interfaces! Also, as a result we have seen a distinct improvement in iSCSI throughput. This does lead me back as to whether a managed switch is best practice for iSCSI. I may write another article about why an unmanaged switch may be the best option for use as an iSCSI switch…

Anyway, if you want to disable QoS on the switch enter the following command (make sure that you are in configuration mode!):

Switch(config)#no mls qos