[c-nsp] PFC for iSCSI on Nexus

[Lincoln Dale]

Tom,

iSCSI runs atop of TCP.  generally speaking, the TCP state machine uses packet drop (lost segments) to tune its transmit rate to the capabilities of the network end-to-end.

PFC will essentially provide a no-drop environment which while in face value may seem to be beneficial in reality it will impair the "goodput" that is possible with many TCP stacks.
TCP is not really tuned for situations where there is no-drop but variable latency as a result of PFC.

where PFC is likely to be beneficial is if you are using a storage protocol which is not based on TCP.  e.g. NFS with UDP.


cheers,

lincoln.

On 10/07/2010, at 2:54 AM, Tom wrote:

> Hello,
> Based on what I've read PFC (Priority Flow Control) just does a
> "pause" type functionality on a more granular level, per CoS instead
> of a per link. I was wondering if you could configure PFC policy for
> iSCSI on the Cisco Nexus 5000s or is that only hard written for only
> FCoE? Not sure how that would work with the TCP transmit queues but
> should create a lossless functionality for iSCSI.
> 
> Anyone has a Nexus 5000 and played around with the PFC configuration?