RE: iscsi : iscsi parameter default values

[Robert Snively <rsnively@Brocade.COM>]

Julian,

For SCSI Write operations, ImmediateData = yes is 
the most demanding mode for the target.  If I understand
it correctly, it essentially over-rides the R2T state, 
allowing the first burst to be included as immediate data.
In SCSI, except for special cases like this, the target 
is the device in charge of data transfers.

This mode requires the target to have a guaranteed collection of
received buffer space adequate to receive all possible
outbound queued operations from all possible initiators 
through all possible target sessions (which may sum to 
1000s of output operations), regardless of what other
buffer-intensive operations may be going on in the device.

It then requires the device to provide association with each
of those commands instead of just the commands it has elected
to activate.  Without immediate data, the command buffer can be
separated and separately managed from the data buffer, 
limiting buffer requirements.

It then requires the device to operate in a non-zero-copy
mode (which raises buffer utilization and increases latency)
while the device analyzes the command to determine what should
be done with the data.  It further requires the subsequent
data (if there is more than one PDU to be transmitted) to
be separately managed, and perhaps actually stored in a
separate operation if the buffer must be cleared to make
space available for it.  It further requires the target to
analyze the data for completeness before going on to determine
what to do with it.

Sure, it is easy for the initiator, but so is everything else
except out-of-order reads.
It is the target you are stressing with this. 

For local sub-LANs, and depending on the actual buffering
available in the target, the performance may actually be lower with
ImmediateData allowed, because zero copy behavior of the 
target to non-volatile media is not available, which raises
buffer utilization.

Have I missed something that would change my mind?

Bob


> -----Original Message-----
> From: Julian Satran [mailto:Julian_Satran@il.ibm.com]
> Sent: Friday, September 28, 2001 10:38 AM
> To: ips@ece.cmu.edu
> Subject: RE: iscsi : iscsi parameter default values
> 
> 
> 
> 
> Robert,
> 
> I disagree that Immediate is "the most demanding" mode.  That 
> is not my
> experience and not what I heard from most of the developers.  On the
> contrary immediate data do not require a separate indexing 
> operation on the
> target (as non-immediate unsolicited do) and that was the base for the
> consensus to have them negotiated separately.
> 
> For the software initiator it is the most inexpensive way to 
> perform short
> data transfers (4-8k) typical for major applications like 
> Database access
> and so it is for lightweight target.
> 
> Julo
> 
> 
> 
>                                                               
>                                    
>                     Robert Snively                            
>                                    
>                     <rsnively@broc       To:     John 
> Hufferd/San Jose/IBM@IBMUS, Julian         
>                     ade.com>              
> Satran/Haifa/IBM@IBMIL                                 
>                                          cc:     
> ips@ece.cmu.edu                                 
>                     28-09-01 17:25       Subject:     RE: 
> iscsi : iscsi parameter default values 
>                     Please respond                            
>                                    
>                     to Robert                                 
>                                    
>                     Snively                                   
>                                    
>                                                               
>                                    
>                                                               
>                                    
> 
> 
> 
> I vote no as the default value on ImmediateData.
> 
> A default value of yes on ImmediateData requires implementation
> of the most complex and demanding mode of operation as the
> default.  SCSI has traditionally made its default behavior the
> simplest and most encompassing, setting more sophisticated
> behavior by subsequent agreement.  While it may be your earnest
> desire to encourage the implementation of this function, it
> is not appropriate to demand that as the default behavior
> of all devices.  In particular, there is no special benefit
> to providing ImmediateData in low-cost local sub-lans.
> 
> If you want to encourage it in a profile, fine, but demanding
> it as the default in the core standard is not appropriate.
> 
> Note that the behavior of SCSI is traditionally managed
> entirely by the target.  As such, there has never before now
> been a requirement for the target to, as a default, accept
> any PDU except a command or task management function
> that was not explicitly solicited.  That is one of the mechanisms
> that assists SCSI in achieving a low-overhead zero copy
> capability while operating with a large number of initiators
> and with deep command queues.
> 
> Bob Snively                        e-mail:    rsnively@brocade.com
> Brocade Communications Systems     phone:  408 487 8135
> 1745 Technology Drive
> San Jose, CA 95110
> 
> 
> 
> > -----Original Message-----
> > From: John Hufferd [mailto:hufferd@us.ibm.com]
> > Sent: Friday, September 28, 2001 1:33 AM
> > To: Julian Satran
> > Cc: ips@ece.cmu.edu
> > Subject: RE: iscsi : iscsi parameter default values
> >
> >
> >
> > I also agree with this.  It should be yes.
> >
> > .
> > .
> > .
> > John L. Hufferd
> > Senior Technical Staff Member (STSM)
> > IBM/SSG San Jose Ca
> > Main Office (408) 256-0403, Tie: 276-0403,  eFax: (408) 904-4688
> > Home Office (408) 997-6136
> > Internet address: hufferd@us.ibm.com
> >
> >
> > Julian Satran/Haifa/IBM@IBMIL@ece.cmu.edu on 09/27/2001 09:50:21 AM
> >
> > Sent by:  owner-ips@ece.cmu.edu
> >
> >
> > To:   ips@ece.cmu.edu
> > cc:
> > Subject:  RE: iscsi : iscsi parameter default values
> >
> >
> >
> >
> > The one that I have trouble living with is ImmediateData.
> > This is important
> > for low-end desktops and hardly matters for large boxes.
> > As such I would suggest it stays as yes.
> >
> > Julo
> >
> >
> >
> >                     "Eddy
> >                     Quicksall"            To:     "'Santosh Rao'"
> > <santoshr@cup.hp.com>,
> >                     <EQuicksall@med        <ips@ece.cmu.edu>
> >                     iaone.net>            cc:
> >                     Sent by:              Subject:     RE:
> > iscsi : iscsi
> > parameter default values
> >                     owner-ips@ece.c
> >                     mu.edu
> >
> >
> >                     27-09-01 17:22
> >                     Please respond
> >                     to "Eddy
> >                     Quicksall"
> >
> >
> >
> >
> >
> > I like your defaults below.
> >
> > But, section 5 says:
> >
> >  The initial Login request MUST include the InitiatorName and
> >  SessionType key=value pairs.
> >
> > Since SessionType is REQUIRED, naming a default would imply a
> > possible typo
> > in the spec.
> >
> > Eddy
> >
> > -----Original Message-----
> > From: Santosh Rao [mailto:santoshr@cup.hp.com]
> > Sent: Wednesday, September 26, 2001 10:29 PM
> > To: ips@ece.cmu.edu
> > Subject: iscsi : iscsi parameter default values
> >
> >
> > All,
> >
> > With the issue of mode page vs. login keys having [almost] 
> drawn to a
> > close, I would like to
> > raise the below issues again, on the subject of default
> > values for login
> > keys and other iscsi
> > parameters :
> >
> >
> >    * In keeping with traditional use of scsi mode pages, 
> iscsi should
> > not specify any default
> >      settings for any mode pages that continue to exist for iscsi.
> > "Default values" for mode
> >      pages are target specific and should not be bound to 
> the protocol
> > draft.
> >
> >    * MORE IMPORTANTLY, I read the default for EMDP as being set to 1
> > :-<  This implies that
> >      targets must be always prepared to deal with out of
> > order data and
> > initiators must be
> >      prepared to deal with out of order data, unless the initiator
> > performs a mode select to
> >      disable it. [which defeats all the previous advantages
> > gained thru
> > mandating use of login
> >      keys for other negotiations.]. A default, if it were to exist,
> > should be 0. (in-order, by
> >      default).
> >
> >    * Conservative specification of defaults for login keys along the
> > following lines :
> >                             MaxConnections = 1
> >                             FMarker = "no"
> >                             InitialR2T = "yes"
> >                             BidiInitialR2T = "yes"
> >                             ImmediateData = "no"
> >                             DataPDULength = 16
> >                             MaxOutstandingR2T = 1
> >                             DataPDUInOrder = "yes"
> >                             ErrorRecoveryLevel = 0
> >                             SessionType = "normal"
> >
> >    * Should the iscsi protocol require a "Lun Control Mode 
> Page"? IOW,
> > is an EnableCRN bit
> >      required at the transport layer ? If the device server 
> capability
> > is to be negotiated , I
> >      suggest this bit be moved to a SCSI ULP Mode Page such as the
> > "Control Mode Page", through a
> >      T10 change as a part of the SCSI changes being driven by iscsi.
> >
> > Comments ?
> >
> > Thanks,
> > Santosh
> >
> >
> > Santosh Rao wrote:
> >
> > > There are the separate issues of :
> > >
> > >    * iscsi's specification of defaults for its mode pages
> > which is not
> > in line with mode page
> > >      usage. This impacts the target's ability to enforce
> > values other
> > than the protocol
> > >      specified default, if the initiator were to not use mode
> > sense/select.
> > >
> > >    * default settings for login keys.
> > >
> > >    * Is there a need for the "LUN Control Mode Page" and whether
> > "Enable CRN" should be in a
> > >      transport specific mode page ?
> > >
> > > which need to be driven to closure as well.
> > >
> > > Regards,
> > > Santosh
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> 
> 
> 
> 
>