Re: Avoiding deadlock in iSCSI

["Jim Williams" <jimw@giganet.com>]

Layering iSCSI on VI avoids the problem below.

see:
http://search.ietf.org/internet-drafts/draft-csapuntz-ips-iscsivi-00.txt

VI layer flow control will be invoked when target is unable to accept more
commands, but this will not impede flow of data in response to RDMA read
requests by the target.  Functionally this is equivalent to option 1 below.


-----Original Message-----
From: csapuntz@cisco.com <csapuntz@cisco.com>
To: ips@ece.cmu.edu <ips@ece.cmu.edu>
Cc: csapuntz@cisco.com <csapuntz@cisco.com>
Date: Monday, September 11, 2000 6:14 PM
Subject: Avoiding deadlock in iSCSI


>
>The problem:
>
>iSCSI, as currently spec'ed, allows SCSI commands and data to be
>interleaved fairly freely on a TCP connection. A target that stops
>reading from a TCP connection to avoid reading more command packets
>also prevents itself from reading data packets.  Those data packets
>may be criticial to making progress on the currently executing
>command.
>
>Note the issue appears with one TCP connection for control and data
>and even appears in many of the multiple connection schemes.
>
>Data in iSCSI comes in two forms:
>
> 1) solicited - data requested by target via RTT
>              - data requested by initiator via a SCSI command
> 2) unsolicited - data sent by initiator without having received an RTT
>
>The analysis below assumes that unsolicited data travels over the same
>TCP connection as SCSI commands. Otherwise, you run the risk of receiving
>unsolicited data before the relevant SCSI command (thus making
>implementations more complex).
>
>Four solutions:
>
>1) Don't overflow the command queue (i.e. use credits)
> - and what do you do if a misbehaving initiator overflows
>        your command queue anyway? Drop the connection?
>
> - requires you to reserve resources per initiator. some people
>        may want to overcommit
>
>2) Allow dropping of SCSI commands when queue fills
> - how do you clean up after a dropped SCSI command?
>     - there may be other commands in the pipeline
>
> One approach: On command drop, the target enters an error
> state. While in the error state, all newly received commands
> terminate with an error until the initiator explicitly clears
> the error state using a "clear error state" message.
>
> You might think that TASK SET FULL and ACA mechanisms from SCSI
>        could be used to attack this problem. However, TASK SET FULL errors
> don't trigger ACA (in my reading of the SAM). Also, ACA is only
> triggered by the current enabled command, not by random commands
> entered into the task set.
>
>3) Put solicited data on a dedicated TCP connection. Require that
>unsolicited data MUST follow the command, ideally in the same iSCSI
>PDU
>
>4) (Do it like NFS) Make all transfers from initiator to target
>unsolicited. Make sure unsolicited data follows the command
>immediately.
>
>
>Of all the options, #1 and #4 sound the easiest to implement. #2 is more
>sophisticated than #1. #3 is just plain clever but that's rarely a good
>thing. :)  #4 has large ramifications on current SCSI target designs.
>
>-Costa
>