RE: ISCSI: flow control

[Black_David@emc.com]

Mike,

> In essence, this is what InfiniBand does and others have been 
> advocating.  When the ACK (SCSI response) is returned it encodes a credit 
> to inform the sender of how many receives buffers (available command queue

> slots) have been posted. 

Could you post, or provide a pointer to a self-contained specification of
that
mechanism?  If this is a pointer to InfiniBand specs, a heads-up on any
intellectual property issues is in order.

A concern that has been raised in this discussion is how
credit information relates to the concurrency and ordering (esp. lack
thereof) of transmission and processing of SCSI commands and
the transmission of responses.  My understanding of the FCP approach
to buffer management (and I assume InfiniBand is similar) is that traffic
cannot be sent unless the sender knows that there is space in the receiver's
buffer to accommodate it (i.e., the sender has a credit or credits
indicating
space in the receiver's buffer).  This implies is that if for some reason
the receiver stalled, all the in-flight commands and data could be
successfully
received.  In contrast, I've seen discussion on this list of long distance
connections in which there is potentially more traffic in flight than the
receiver
could accommodate if the receiver stopped.  I believe that whether to allow
this is an open issue, but the underlying cause is valid - there is a desire
to use iSCSI in situations where the initiator to target coupling is looser
(in this case, due to distance) than is typical for SCSI and Fibre Channel.

--David

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David L. Black, Senior Technologist
EMC Corporation, 42 South St., Hopkinton, MA  01748
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