Re: iSCSI: Out of order commands

["Mallikarjun C." <cbm@rose.hp.com>]

Rod and Julian,

This has been an interesting thread of discussion.  Some
comments -

1.My first reaction was - allowing out-of-order command
  transmission on the same connection deprives targets of
  an implementation choice.  Targets which support only 
  single-connection sessions and only support session 
  recovery (reasonable assumptions in my mind) can no 
  longer afford *not to* implement a command scoreboard.

2.Any end-node efficiency that is sought to be achieved
  by transmitting CmdSNs out-of-order from the initiator
  would be lost on the other end-node, since the target
  now must wait for re-ordering the commands.  

3.The flipside is that out-of-order transmission saves 
  link badwidth (albeit at the expense of end-node efficiency),
  compared to idling the link waiting for outbound DMA. 
  We have to determine if this is a reasonable trade-off.

4.I can see Rod's point that prefetching all immediate 
  data can be a burden on the NIC resources.  But, two
  questions -
	- could the NIC not use unsolicited separate data
          PDUs in these cases? [ I realize that InitialR2T
          has to be "no" to let it happen... ]
	- could the NIC have a memory architecture that 
          allows data prefetching for the next command (so
          this is a non-issue from the protocol perspective)?
          This scheme incurs one DMA delay for every new 
          burst of commands.

5.Another (perhaps radical at this point) option is to do 
  away with immediate unsolicited data, to stick only with
  separate unsolicited data.  I would personally be okay
  with the choice, particularly if this feature (that 
  helps software implementations) starts making hardware
  design complicated/expensive. 

So, to summarize -


option                         immediate         allow
                               data in spec?     out-of-order?

(A) (5) above                  no                no
(B) No real reason to do this. no                yes
(C) (4) above                  yes               no
(D) pros & cons (1), (2) & (3) yes               yes

From the arguments I heard so far, I am leaning towards
option A, and option C in that order.

Comments?
-- 
Mallikarjun 


Mallikarjun Chadalapaka
Networked Storage Architecture
Network Storage Solutions Organization
MS 5668	Hewlett-Packard, Roseville.
cbm@rose.hp.com


Rod Harrison wrote:
> 
> Julian,
> 
>         I don't understand what you are proposing here, what do you mean by
> "multiplexed" DMA?
> 
>         The problem is that the DMAs take some time, the more there are
> queued the longer the last DMAs queued take to complete. Some commands
> require DMAs to complete before they can be sent, i.e. Writes with
> immediate data, some commands do not, i.e. Reads and writes with no
> immediate data. The iSCSI HBA wants to be able to send commands as
> soon a possible, which for a read after a write can be before the
> write's DMA has completed. Maintaining an ordered queue for commands
> to be sent on the HBA is expensive and redundant since the target
> already knows how to queue commands before committing them to its SCSI
> layer.
> 
>         The iSCSI HBA and its host driver are not at liberty to change the
> order of commands from the OS, but the DMAs those commands need are
> unlikely to complete in the same order, and as I mentioned some
> commands need no DMA. If the HBA can't send commands out of CmdSN
> order it has to maintain an ordered queue of commands waiting to be
> sent, and potentially buffer a lot of data. For an HBA this makes
> immediate data almost impossible to support.
> 
>         I don't see the problem with allowing out of order commands given
> that the target already has to deal with very similar problems. I
> think we are getting in to the area of implementation choices here,
> which is inappropriate for a specification.
> 
>         - Rod
> 
> -----Original Message-----
> From: owner-ips@ece.cmu.edu [mailto:owner-ips@ece.cmu.edu]On Behalf Of
> Julian Satran
> Sent: Monday, November 05, 2001 10:06 PM
> To: ips@ece.cmu.edu
> Subject: Re: iSCSI: Out of order commands, was current UNH Plugfest
> 
> Rod,
> 
> I don't see any reason why DMA operations cant be "multiplexed" with
> commands.
> If you have scheduled a long outbound DMA you are doomed regardless of
> the
> command ordering.
> And if you have scheduled DMA operations piecemeal then you can insert
> your commands in correct order.
> 
> Julo
> 
> "Rod Harrison" <rod.harrison@windriver.com>
> 05-11-01 20:48
> Please respond to "Rod Harrison"
> 
>         To:     Julian Satran/Haifa/IBM@IBMIL, <ips@ece.cmu.edu>
>         cc:
>         Subject:        iSCSI: Out of order commands, was current UNH
> Plugfest
> 
>                  [ Subject changed ]
> 
> Julian,
> 
>                  The ordering difference is introduced between the
> host
> side driver
> and the iSCSI HBA. The host side driver must present SCSI commands to
> the HBA in the order they are received from the OS to prevent read
> after write dependency failures. The HBA might reorder the commands
> depending on when DMA completes. The reordering can't be done ahead of
> time in the host driver since it doesn't know how long each DMA might
> take. As long as the HBA assigns CmdSN in the order it receives
> commands the desired host ordering is preserved.
> 
>                  - Rod
> 
> -----Original Message-----
> From: owner-ips@ece.cmu.edu [mailto:owner-ips@ece.cmu.edu]On Behalf Of
> Julian Satran
> Sent: Monday, November 05, 2001 12:35 AM
> To: ips@ece.cmu.edu
> Subject: RE: iSCSI: current UNH Plugfest
> 
> Rod,
> 
> I all examples give the point I find hard to understand is why is the
> ordering on the wire different from the presentation order to the
> initiator.  You can get as many overlaps as you want by presenting the
> commands to the initiator in the desired order.
> What we are considering here is the case in which you want to ship in
> an
> order different than the one you present the commands.
> 
> Julo
> 
> "Rod Harrison" <rod.harrison@windriver.com>
> Sent by: owner-ips@ece.cmu.edu
> 04-11-01 04:42
> Please respond to "Rod Harrison"
> 
>         To:     "Barry Reinhold" <bbrtrebia@mediaone.net>, "Dave
> Sheehy"
> <dbs@acropora.rose.agilent.com>, "IETF IP SAN Reflector"
> <ips@ece.cmu.edu>
>         cc:
>         Subject:        RE: iSCSI: current UNH Plugfest
> 
> Barry,
> 
>                  In general I agree but I don't think this is as much
> of a
> corner case
> as it at first appears. Targets will have code very similar to that
> needed to handle out of order commands to deal with digest errors.
> Targets also need to queue commands whilst waiting for both solicited
> and unsolicited data to arrive. Queuing out of order commands seems
> little extra work.
> 
>                  From an initiators point of view there are
> efficiency,
> and probably
> performance gains to be had from sending commands out of order. Bob
> Russell gave the example of a read being sent whilst write data DMA is
> happening, and a similar situation can arise with DMA for writes
> overtaking that of earlier writes if the initiator has multiple DMA
> engines. In this case the initiator might be forced to let the wire go
> idle if it can't send the data from completed DMAs as soon as
> possible.
> 
>                  We already have a command queue at the target to
> enforce
> correct
> serialisation of commands, doing the same thing at the initiator is
> redundant.
> 
>                  Finally, I don't believe we should be writing a
> standard
> to work
> around poor coding and test coverage, especially at the cost of
> potential efficiency gains.
> 
>                  I agree with Dave and Santosh that commands being
> sent
> out of order
> on a single session should be allowed by the standard.
> 
>                  - Rod
> 
> -----Original Message-----
> From: owner-ips@ece.cmu.edu [mailto:owner-ips@ece.cmu.edu]On Behalf Of
> Barry Reinhold
> Sent: Friday, November 02, 2001 5:24 PM
> To: Dave Sheehy; IETF IP SAN Reflector
> Subject: RE: iSCSI: current UNH Plugfest
> 
> Using features such as out of order command delivery on a connection
> tend to
> be the sort of things that lead to interoperability problems. It is
> unexpected and probably going to hit poorly tested code paths even if
> the
> standard is written to allow it.
> 
> >-----Original Message-----
> >From: owner-ips@ece.cmu.edu [mailto:owner-ips@ece.cmu.edu]On Behalf
> Of
> >Dave Sheehy
> >Sent: Friday, November 02, 2001 4:19 PM
> >To: IETF IP SAN Reflector
> >Subject: Re: iSCSI: current UNH Plugfest
> >
> >
> >
> >> 3. Can commands be sent out of order on the same connection?
> >>
> >>    The behavior of targets is clearly specified in Section 2.2.2.3
> on
> >>    page 25 of draft 8, which says:
> >>      "Except for the commands marked for immediate delivery the
> iSCSI
> >>      target layer MUST eliver the commands for execution in the
> order
> >>      specified by CmdSN."
> >>
> >>    Section 2.2.2.3 on page 26 of draft 8 also says:
> >>      "- CmdSN - the current command Sequence Number advanced by 1
> on
> >>      each command shipped except for commands marked for immediate
> >>      delivery."
> >>    but the meaning of the term "shipped" is vague, and does not
> >> necessarily
> >>    require that the PDUs arrive on the other end of a TCP
> connection
> >>    in the same order that the CmdSN values were assigned to these
> PDUs.
> >>
> >>    Some initiators have been designed to send commands out of CmdSN
> >>    order on one connection.  Consider the situation where there is
> only
> >>    one connection and a high-level dispatcher creates a PDU for a
> SCSI
> >>    command that involves writing immediate data to the target.
> This PDU
> >>    is enqueued to a lower-level layer which has to setup, start,
> and
> >>    wait-for a DMA operation to move the immediate data into an
> onboard
> >>    buffer before the PDU can be put onto the wire.  While this is
> >>    happening, the dispatcher creates another unrelated PDU for a
> SCSI
> >>    read command (for example), and when this PDU is passed to the
> >>    lower-level layer it can be sent immediately, ahead of the
> previous
> >>    write PDU and therefore out of order on this connection.
> >>
> >>    The standard clearly allows this to happen if the two PDUs were
> sent
> >>    on different connections, and seems to imply that this can also
> happen
> >>    when the two PDUs are sent on the same connection.
> >>
> >>    The suggestion is to put in the standard an explicit statement
> that
> >>    this is allowed or not allowed, as appropriate.
> >>
> >>    If this is allowed, such a statement would avoid the erroneous
> >>    assumption being made by some target implementers that within a
> single
> >>    connection, commands will arrive in order.
> >>
> >>    If this is not allowed, such a statement would avoid the
> erroneous
> >>    assumption being made by some initiator implementers that within
> a
> >>    single connection, commands can be put on the wire out of order.
> >>
> >> +++
> >>
> >> will add an explicit statement saying that this behaviour is
> forbidden.
> >> 2.2.2.1 will contain:
> >>
> >> On any given connection, the iSCSI initiator MUST send the
> >commands in the
> >> order specified by CmdSN.
> >>
> >> +++
> >
> >Why do you feel this behavior should be forbidden? Targets already
> have to
> >order commands across the session. I don't see why it's a problem to
> extend
> >that to the connection as well. I, for one, believe we should take
> >a liberal
> >stance on this.
> >
> >Dave Sheehy
> >