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Flow Control Question

Posted on 2011-02-23
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Last Modified: 2012-05-11
I have read that Flow Control is important to have enabled on iSCSI networks.  I am confused as to where it needs to be enabled though.

I know I have to enable it on VMWare (not sure if it is RX, TX, or both though)
http://www.sysadmintutorials.com/tutorials/vmware-vsphere-4/esx-4/enabling-flow-control-with-vmware-esx-4/

And I know how to enable it on the switch (HP 2910al)

The SAN (Enhance Tech ES3160) does not have any Flow Control configuration though.  

Is this something I only enable on the switch and host?  Or can I not use it b/c the SAN doesn't support it?
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Question by:jasonrac
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rfc1180 earned 2000 total points
ID: 34964163
If you do not already know, flow control is used to Generate & Respond on all NICs to allow hardware-based adjustments that prevent dropped packets. Setting flow control is highly recommended and helps resolve in an efficient manner any imbalance in network traffic between sending and receiving devices; helps in situations where microburst of traffic occure.

You will want to configure flow control on each of the switchports that handles iSCSI traffic.  If the server is using a software iSCSI initiator and network interface card (NIC) combination to handle iSCSI traffic, flow control must also be enabled on the NICs to obtain the performance benefit.

Additionally, configure jumbo frame (All devices on the network must support jumbo frames and must match the size of the frame end to end) on each switch that handles iSCSI traffic. If a server is using a software iSCSI initiator and NIC combination to handle iSCSI traffic, enable jumbo frames on the NICs to obtain the performance benefit and help ensure consistent behavior.

Billy
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Author Comment

by:jasonrac
ID: 34964283
Thank you -

So enabling flow control on the NICs in VMWare (I am using the Software iSCSI Initiator) and the Switch is the correct thing to do.

One question here - according to the article I linked to above I can set the flow control to Rx, Tx, or Both - which should I set the NICs to?

Also - on the switchports that connect to the SAN should I have flow control turned off (I think so but want to check)

I have enabled Jumbo frames on everything - thank you.
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LVL 24

Expert Comment

by:rfc1180
ID: 34964427
>So enabling flow control on the NICs in VMWare (I am using the Software iSCSI Initiator) and the Switch is the correct thing to do.

correct

>according to the article I linked to above I can set the flow control to Rx, Tx, or Both - which should I set the NICs to?

both

Also - on the switchports that connect to the SAN should I have flow control turned off (I think so but want to check)

I do

Billy
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Author Comment

by:jasonrac
ID: 34964537
Sorry one last question - I do means Flow Control off on the SAN switch ports?
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LVL 24

Expert Comment

by:rfc1180
ID: 34964789
Correct, the SAN is the device that receives the data, so enabling flow control at times on the SAN switchports can do more harm than good depending on small/large the iSCSI network is. I leave my disable (Flow control) and have not had any issues.

Billy
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Author Comment

by:jasonrac
ID: 34965033
Great - thank you again!
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