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iSCSI through Virtual Connect with Flow Control

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iSCSI through Virtual Connect with Flow Control

Nick had some questions regarding iSCSI and VC FlowControl settings




During my current ESXi implementation the customer gave me the NetApp iSCSI best practices guide and said “here’s how I want the iSCSI set up”. There wasn’t much in there that was a surprise except around iSCSI flow control.


In the HP iSCSI guide it states that Flow Control should be on across all the interfaces from the server to the SAN.


However, I the NetApp guide has the following;



Modern network equipment and protocols generally handle port congestion better than in the past. While NetApp had previously recommended flow control ―send‖ on ESX hosts and NetApp storage controllers, the current recommendation, especially with 10 Gigabit Ethernet equipment, is to disable flow control on ESXi, NetApp FAS, and the switches in between.


My concern is that we only have 2 x 10GB connections out of the whole chassis, 1 each in 2 active/active Shared Uplink Sets. All traffic, (iSCSI, client, vMotion etc.) exits via these 2 links. So if we disable flow control on the VC domain all traffic will lose flow control.


The chassis itself is a c3000 and will not be that heavily loaded, initially with only 2 x ESXi (v5) hosts and potentially about 20 VMs, and maybe a physical SQL server at a future date.


Any ideas/thoughts? I have been reading a few articles but nothing really clears it up. Should I go with the NetApp recommendation and disable flow control in VC (Flex 10), or go with the HP recommendation and leave flow control enabled? The NetApp is currently set for no flow control.


We are not using accelerated iSCSI, just the ESXi software initiators.




Input from Chris:




Flow Control is only going to come into play when there is port congestion.  I would suggest your customer run some IOMETER tests to determine the appropriate Flow-Control setting.  As you stated, Flow-Control is enabled in VC by default.




Any other comments?