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In the testing/playing phase with a bunch of new SAN/Server kit.

I have 2 x Procurve 2910 switches dedicated to iSCSI/vMotion (with a 10gbps fibre link between them), 2 x vSphere hosts (each with lots of NICs), and 6 x P4000 iSCSI nodes.

The P4000 manuals say to enable flow control on the switches.

I'm not clear if that means enable it globally, or just on the ports that the P4000 nodes are directly connected to?

I'll be mounting iSCSI volumes from both the vSphere hosts, and from within some of the guests (Windows 2008 DSM MPIO).

I know what flow control does at a basic level, but I don't know enough about it at low level to know where I should be implementing it.


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Depends on the switch, I know on Juniper switches flow control is a per port option. You MAY need to enable it on the NIC of the host too, but most NICs have it turned on by default. – SpacemanSpiff Dec 12 '10 at 14:37
You're probably also going to want to enable jumbo frames. – SpacemanSpiff Dec 12 '10 at 14:39
Thanks Tom, to clarify, I know on the Procurve's you can enable it per port OR for the entire switch, but I don't know enough about Flow Control to know which is the right way to do it, if that makes sense, as if I enable it on the switch it's there for anything/everything, but if I only enable it on selected ports, which ports... and Jumbo Frames on the iSCSI VLAN possibly yes, that's step 2 :) – Hutch Dec 12 '10 at 14:42
up vote 1 down vote accepted

Per port should be sufficient provided you are covering all of the iSCSI connections to the switch - that's both those connecting to the P4000 nodes and any to hosts that are consuming the traffic. In most cases though enabling it for the entire switch would be desirable because you want Flow Control operating for all ports handling iSCSI traffic. If you have any devices that can't handle flow control then try to fix that or move them to a separate switch if possible. As Tom said in his comments most Nics will have it enabled by default.

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Ah I just checked some of the NIC drivers of my existing servers. The vNICs on VM's don't have a Flow Control setting. The NICs on my physical Windows boxes do, and they're set to "Auto". I believe from the vmware KB that pNICs on vsphere are also set to "Auto". – Hutch Dec 12 '10 at 15:50
The pnics are the ones that matter if you are delivering iSCSI storage to the vSphere hosts and they are set to auto however for the VM's that you are directly presenting storage to the native iSCSI you may need to use the e1000 vnic to provide flow control. – Helvick Dec 12 '10 at 16:37
Thanks Helvik, I'll look into that - everything here is brand new kit so it's 1gbps or 10gbps end-to-end so the only reason I'm looking specifically at flow control is because the P4000 guide suggests it's best practise to enable it. – Hutch Dec 12 '10 at 18:08

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