RE: Avoiding deadlock in iSCSI

[Jim McGrath <Jim.McGrath@quantum.com>]

I think people have been meaning "unsolicited data" to really mean data sent
to a receiver without that receiver having first indicated that there is
enough buffering to hold the data.  For initiators acting as receivers they
have to verify this before they initiate the command (not enough space for
the whole command?  Then break up the command.)  For Targets this requires
something like a credit mechanism with RTTs being used.

So there is an "unsolicited command" problem and an "unsolicited data"
problem.  In both cases the sender creates the problem by not first
reserving with the receiver enough resources for the commands/data.
  
In the command case there is no SCSI mechanism to reserve resources (QUEUE
FULL is used to indicate overflows).  Historically it has been assumed that
queues of commands do not overflow often in practice.  In reality initiators
have often artificially limited the number of commands they are willing to
try and queue at the target in order to avoid this rejection (a loss
opportunity in my mind).

In the data case there is no "DATA QUEUE FULL" - instead, an explicit credit
model of some sort is used to indicate the receiver has reserved space for
the data (REQs in parallel SCSI, BB credits in Fibre Channel).  In this case
the assumption was that data overflows would occur a lot otherwise.

You can solve these problems by rejecting the overflow cleanly (as SCSI does
with commands), which is low latency and works well under light loads.  Or
you can do credits.  Credits add latency, or get you into the problem of
credit allocation, which can be optimized for light load (over allocate
credits) or heavy loads (allocate only what you have), but not both at once.

Historically, SCSI has used rejection for commands and credits for data,
optimized for heavy loads.  But this is only a T10 given rule, not a God
given rule (although some of us who have served on T10 can get that confused
at times :-)).

Hope this helps.

Jim





cases there are well known mechanisms to reserve the 

-----Original Message-----
From: David Robinson [mailto:David.Robinson@EBay.Sun.COM]
Sent: Monday, September 11, 2000 3:35 PM
To: ips@ece.cmu.edu
Subject: Re: Avoiding deadlock in iSCSI


I think in following this discussion the terminology has been
confusing me.  When I read "unsolicited data" I interpreted that
to mean data for which no command has yet been sent. In general
I consider that to be a bug and the receiver should just drop the
data on the floor.  The only possible scenerio where it might
not be a bug is if a command was sent on one connection and the
data on the data connection arrived first, thus it is unsolicited.
My first assumption is that the sender would not send commands
C1 and C2 and data D2 and D1 on the same connection. Doing that
creates nasty ordering problems we want to avoid.  So if the
receiver simply allows the data connection TCP window to shrink
the unsolicted data will flow control to a stop until the command
queue catches up.  With multiple data connections, some may flow
control but the active command will be able to make progress on
one connection. This may not be the most efficient mechanism but
it is "safe".  Preferably the data will either follow the command
on the same data/command connection or the sender will request a
RTT (aka R2T). It is also a sender bug to request a connection
for data transfer that it has already sent "unsolicited" data.

Unless my assumptions and definitions are wrong, I don't see the issue.

	-David
	
> 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