iSCSI is encapsulated in TCP.  TCP provides for variable size transmission windows.  So if a packet is lost, the default behaviour is to reduce the window size.  Plus, the iSCSI layer could specifically request the window be increased or decreased.


Tom


On Apr 17, 2015, at 11:03 AM, basilberntsen@gmail.com wrote:

I have virtually no iscsi experience, but how would upper layers control flow? Is that at the iscsi driver level?

Sent from my BlackBerry 10 smartphone on the Bell network.
From: tmac
Sent: Friday, April 17, 2015 7:44 PM
To: Philbert Rupkins
Subject: Re: Excessive Flow Control Frames?

Realize that every time the switch sends the Filer a pause frame
*all network traffic* out that interface ceases until Xon come in.

In that regard, any pause frames can slow down your filer.
Turn Flow control off. at least on your netapp. Then it will not accept (or should not!) the pause frames from the switch.

Allow the upper layers to control congestion.

--tmac

Tim McCarthy
Principal Consultant


On Fri, Apr 17, 2015 at 1:22 PM, Philbert Rupkins <philbertrupkins@gmail.com> wrote:
Toasters,

We have ethernet flow control set to full on our NetApp's 10g interfaces.    The FEX ports to which our 10g interfaces are connected also have flow control TX/RX enabled.   I realize enabling flow control is against best practices. 

I am trying to determine if we are seeing an excessive number of flow control frames.   We received over 12,000 PAUSE frames from the FEX within a 10 minute period which sounds excessive to me.   Can anybody confirm that 12,000 is an excessive number of PAUSE frames that would likely result in connectivity issues?

Here are statistics for one of our 10g interfaces during a recent 10 minute interval.  Notice that the NetApp is receiving PAUSE frames (RECEIVE-Xoff) from the FEX and never transmitting (TRANSMIT-Xoff).

RECEIVE
 Jabber:              0  | Bus overruns:        0  | Xon:             12412
 Xoff:            12412  | Jumbo:               0
TRANSMIT
 Queue overflows:     0  | No buffers:          0  | Xon:                 0
 Xoff:                0  | Jumbo:               0  | TSO non-TCP drop:    0

What I see is the FEX telling the NetApp to slow down.  I just dont know if the FEX is slowing down the NetApp enough that it would cause problems.

Have a great day,
Phil

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