RE: Keep-alive traffic (was iSCSI: more on StatRN)

["Y P Cheng" <ycheng@advansys.com>]

> From: Douglas Otis [mailto:dotis@sanlight.net]
> Sent: Wednesday, November 01, 2000 9:25 AM
> A flow control scheme is required beyond TCP as target and
> initiator traffic must be handled separately over the same connection.
> Unfettered, the initiator will consume buffers required by the target
> for completion of commands.  A type of buffer to buffer control is
> required beyond TCP.

For those of us who design adapters, we punt the flow control problem to
ULP.  For an adapter, ULP must pre-allocated buffers for receiving incoming
segments.  For flow control, I think you are referring the buffers used by
TCP/IP, RAID or File Systems.  Without flow control, when incoming TCP
segment overflows, an adapter just throws them away.

> Presently, there is an optional command window which can provide only
> rudimentary control with respect to a common medium.  Buffer to buffer
> credits for the underlying media allows specific control where
> needed. It is also important to allow multiple targets to be
> carried over the same connection to reduce connection count.
> By having a flow control scheme that resolves to the real target,
> buffers do not become bloated and thus you do not have head of queue
> blocking hiding a SCSI ping.

The adapter treats ACK's and Pings packets separately.  It has the
intelligence to know the ACK of fibre channel.  It could be programmed to
handle iSCSI pings.  As that in TCP-zero-copy, with cooperation from the TCP
driver, the adapter can even be programmed to handle the TCP ACKs without
queuing or waiting.  I guess this is a different way of avoiding blocking.

> Do not forget about the buffers feeding the individual mediums.
> A device being serviced by this server interface is only able to deliver
> about 200 requests per second where most of these requests are small. Only
> by joining together several targets will wire speed be possible
> on any given connection. Unfortunately, iSCSI does not allow this
provision
> and hence requires a connection per target. :(

Yes, to saturate a fibre channel adapter, you need more than 10 fastest
15000 rpm drives.  A TOC-Offload-Engine adapter is capable of running at
wire speed.  But, it relies on ULP to take advantage of this speed.