RE: iSCSI: Out of order commands

["FRYMAN,ROBERT (A-Roseville,ex1)" <robert_fryman@agilent.com>]

> Thus the iSCSI layer delivers C1, C2, C3, and C4. It doesn't worry 
> about the fact that C1 generates data in response to the R2T.
> However the command sequence of C1, C3, C2, C4 forces the iSCSI
> layer to buffer C3. This is significant to a hardware accelerated
> iSCSI layer.

This is significant to a target hardware accelerated iSCSI layer.  By
forcing the initiator to order the iSCSI commands, you move the significants
from the target to the initiator, but it is still a complexity added to a
hardware accelerator.

The problem I have is that we are trying to add a requirement that only
effects a specific area, but adds complexity to a larger area.  If we say
that initiators must order commands on a single connection, then doesn't
this mean that we have just added complexity and slowdown to the enterprise
storage area?

In the Fibre channel realm, a standard base configuration was to have a
target (JBOD, array, etc.) with 2 ports, and an initiator with two ports.
These would be connected on two seperate physical connections, so if a
problem happens on one, the other will take over.  We allow the same thing
in iSCSI, plus specified how the customer can use both connections to
increase bandwidth as long as both connections work.  It seems to me that
any customer that wants a fault tolerant solution, and gets increased
bandwidth for free due to the ability to have multiple connections, would
take advantage of that.

This being the case, these targets have to have reordering capability.  If
we force the initiators to send the comands in order, we just added
complexity and slowdown that adds no benifit.

As I see it (and I could be totally off base here), we have a choice of
complexity and slowdown in the enterprise market, or adding a little
complexity to simple targets.  Am I flawed in this thinking?

Scott Fryman
Agilent Technologies


> -----Original Message-----
> From: Barry Reinhold [mailto:bbrtrebia@mediaone.net]
> Sent: Thursday, November 08, 2001 6:19 AM
> To: Rod Harrison; Robert D. Russell; Somesh Gupta
> Cc: Julian Satran; ips@ece.cmu.edu
> Subject: RE: iSCSI: Out of order commands
> 
> 
> Rod,
> 	I can see your point, from the perspective of a target 
> that is a box
> combining an iSCSI entity with a SCSI entity. However, from 
> the perspective
> of an "iSCSI only" entity, delivery does not have to be 
> delayed until C1
> completes. In this case you only need to ensure that the commands are
> delivered in order to the SCSI layer. The SCSI layer has to 
> address the
> semantics of what the operation entails in terms of command 
> execution. Thus
> the iSCSI layer delivers C1, C2, C3, and C4. It doesn't worry 
> about the fact
> that C1 generates data in response to the R2T. However the 
> command sequence
> of C1, C3, C2, C4 forces the iSCSI layer to buffer C3. This 
> is significant
> to a hardware accelerated iSCSI layer.
> 
> >-----Original Message-----
> >From: Rod Harrison [mailto:rod.harrison@windriver.com]
> >Sent: Wednesday, November 07, 2001 8:34 PM
> >To: Barry Reinhold; Robert D. Russell; Somesh Gupta
> >Cc: Julian Satran; ips@ece.cmu.edu
> >Subject: RE: iSCSI: Out of order commands
> >
> >
> >Barry,
> >
> >	I'm curious, where do you see the extra complexity between the
> >following?
> >
> >	Assuming all the following commands are writes ...
> >
> >c1, c3, c2, c4 and
> >
> >c1 with no payload, c2 + immediate, c3 + immediate, c4 + immediate.
> >
> >	In both cases the target has to queue commands 2, 3, 
> and 4 whilst
> >waiting for its R2T on c1 to be satisfied.
> >
> >	Even if the target doesn't support any unsolicited data 
> it still has
> >to queue commands 2, 3, and 4. I can't see how the target can avoid
> >queuing commands, in which case the order of arrival seems to make
> >little difference.
> >
> >	- Rod
> >
> >-----Original Message-----
> >From: owner-ips@ece.cmu.edu [mailto:owner-ips@ece.cmu.edu]On 
> Behalf Of
> >Barry Reinhold
> >Sent: Wednesday, November 07, 2001 3:10 PM
> >To: Robert D. Russell; Somesh Gupta
> >Cc: Julian Satran; ips@ece.cmu.edu
> >Subject: RE: iSCSI: Out of order commands
> >
> >
> >Bob,
> >	I can't speak for targets, but OOO commands on a session with a
> >single
> >connection sure increases the complexity of the code path we take.
> >	To me the issue is still that these types of situations 
> tend to be
> >poorly
> >tested and lead to interoperability issues. If we do go down this
> >path, the
> >spec. should make it very clear that this is expected behavior. Some
> >statement with a shall in it should be written (for the receiver), so
> >that
> >there is a conformance test items to pass.
> >
> >>-----Original Message-----
> >>From: owner-ips@ece.cmu.edu [mailto:owner-ips@ece.cmu.edu]On Behalf
> >Of
> >>Robert D. Russell
> >>Sent: Wednesday, November 07, 2001 4:13 PM
> >>To: Somesh Gupta
> >>Cc: Julian Satran; ips@ece.cmu.edu
> >>Subject: RE: iSCSI: Out of order commands
> >>
> >>
> >>Somesh, Julian:
> >>
> >>You state that dealing with OOO commands on the target
> >>will add substantial complexity on the target.
> >>Do you have any basis for that claim?  My impression from the
> >>plugfest is that most targets are already dealing with
> >>it.  Perhaps we need to hear from someone who is actually
> >>building a target for which this would be a real problem.
> >>
> >>If anything, what we are hearing from people who really
> >>are building initiators is that dealing with the requirement
> >>to send commands in order will introduce substantial complexity
> >>on the initiator.
> >>
> >>So why should we be saving complexity on (hypothetically) simple
> >>targets yet requiring complexity on real initiators?
> >>
> >>As far as the deadlock issue is concerned, if the only way
> >>that deadlock can occur with OOO commands on the same
> >>connection is during the use of immediate data (which is I
> >>think what Julian was saying), then shouldn't we change
> >>the standard to just say that -- if the initiator sends
> >>commands out of order on a single connection, then immediate
> >>data MUST NOT be used on that connection in order to avoid deadlock.
> >>
> >>This gives everybody what they want, since initiators who find
> >>it beneficial to deliver commands OOO will just negotiate
> >>immediate data off.  Those who really want to use immediate data
> >>will have to ensure that commands are sent in order.
> >>The tradeoff then becomes an implementation issue, not a
> >>standards issue, which is where it belongs.
> >>
> >>
> >>Bob Russell
> >>InterOperability Lab
> >>University of New Hampshire
> >>rdr@iol.unh.edu
> >>603-862-3774
> >>
> >>
> >>On Wed, 7 Nov 2001, Somesh Gupta wrote:
> >>
> >>> I think we should either have it as a MUST or not require
> >>> it (at both ends to get the real benefit). SHOULD is one
> >>> of those things that leads to implementation
> >>> burden and confusion, without perhaps the feature being
> >>> used. There are implementation as well as protocol
> >>> considerations mixed in here.
> >>>
> >>> If we are to remove the restriction, we should (SHOULD)
> >>> get the maximum benefit from it, rather than to
> >>> accomodate an implementation choice. Out of sequence
> >>> commands, combined with the possibility of digest errors,
> >>> will add substantial complexity on the target side,
> >>> without corrosponding benefit in performance. If we change
> >>> this to SHOULD, we should also relax the requirement
> >>> to present commands on the target side to a SHOULD.
> >>>
> >>>
> >>>
> >>> > -----Original Message-----
> >>> > From: owner-ips@ece.cmu.edu [mailto:owner-ips@ece.cmu.edu]On
> >Behalf Of
> >>> > Julian Satran
> >>> > Sent: Wednesday, November 07, 2001 10:00 AM
> >>> > To: ips@ece.cmu.edu
> >>> > Subject: Re: iSCSI: Out of order commands
> >>> >
> >>> >
> >>> > Mallikarjun,
> >>> >
> >>> > I did not see a SINGLE performance improvement that results from
> >OOO
> >>> > shipping.
> >>> > I would be bad engineering to give away the "no-deadlock"
> >mechanism we
> >>> > have now for nothing.
> >>> > I have also the impression that the point about deadlock that I
> >keep
> >>> > repeating is ignored or not understood.
> >>> > As we stand today commands can be shipped with Immediate data
> >>or without
> >>> > and an implementer determined
> >>> > to squeeze maximum bandwidth and overlap command start with
> >>delivery will
> >>> > choose not to work with immediate data
> >>> > (as you have pointed out) while a low performance software
> >>implementation
> >>> > will use immediate data to minimize CPU cycles 
> consumed.  However
> >both
> >>> > will be guaranteed to work without deadlock as source and sink
> >use the
> >>> > same ordering.
> >>> > Recovery is still a low probability event and should be handled
> >with a
> >>> > different set of considerations in mind.
> >>> > As for the strictness of the recommendation - yes we 
> could settle
> >on
> >>> > SHOULD.
> >>> >
> >>> > Julo
> >>> >
> >>> >
> >>> >
> >>> >
> >>> > "Mallikarjun C." <cbm@rose.hp.com>
> >>> > Sent by: owner-ips@ece.cmu.edu
> >>> > 07-11-01 19:41
> >>> > Please respond to cbm
> >>> >
> >>> >
> >>> >         To:     Santosh Rao <santoshr@cup.hp.com>,
> >ips@ece.cmu.edu
> >>> >         cc:
> >>> >         Subject:        Re: iSCSI: Out of order commands
> >>> >
> >>> >
> >>> >
> >>> > Santosh,
> >>> >
> >>> > I have only one comment on your responses.
> >>> >
> >>> > > Even a single connection target *MUST* implement a scoreboard.
> >The
> >>> > > reason being that it can see out-of-order arrival of commands
> >due to
> >>> > > commands being dropped on digest errors. In such a case, it
> >>must block
> >>> > > further command processing until holes are filled.
> >>> >
> >>> > I made two convenient assumptions if you noticed, :-), one of
> >which
> >>> > is that target forces session recovery on *any* error that it
> >sees
> >>> > (ErrorRecoveryLevel=0) - including a dropped command due to a
> >digest
> >>> > error.  With that assumption, a target can afford not to
> >implement
> >>> > a scoreboard.
> >>> >
> >>> > As I said in a private note, I guess what primarily bothers me
> >about
> >>> > OOO commands on a connection is that it requires the receiver to
> >>> > undo this "optimization" on its end - most notably on a single
> >>> > connection.  TCP experts may comment on how/if they dealt with a
> >>> > similar issue.
> >>> >
> >>> > OTOH, you had some valid comments on exceptions to ordering
> >during
> >>> > connection recovery.  Perhaps we can move on by making Julian's
> >>> > proposed stipulation a SHOULD....
> >>> > --
> >>> > Mallikarjun
> >>> >
> >>> >
> >>> > Mallikarjun Chadalapaka
> >>> > Networked Storage Architecture
> >>> > Network Storage Solutions Organization
> >>> > MS 5668          Hewlett-Packard, Roseville.
> >>> > cbm@rose.hp.com
> >>> >
> >>> >
> >>> > Santosh Rao wrote:
> >>> > >
> >>> > > Mallikarjun,
> >>> > >
> >>> > > Some comments below.
> >>> > >
> >>> > > Regards,
> >>> > > Santosh
> >>> > >
> >>> > > "Mallikarjun C." wrote:
> >>> > > >
> >>> > > > 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.
> >>> > >
> >>> > > Even a single connection target *MUST* implement a scoreboard.
> >The
> >>> > > reason being that it can see out-of-order arrival of commands
> >due to
> >>> > > commands being dropped on digest errors. In such a case, it
> >>must block
> >>> > > further command processing until holes are filled.
> >>> > >
> >>> > > Thus, there is no getting away from implementing a 
> sequencer at
> >the
> >>> > > target. Given this, I think it is unreasonable to restrict
> >initiator
> >>> > > implementation flexibility by imposing a strict ordering
> >requirement
> >>> > > within the connection.
> >>> > >
> >>> > > > 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.
> >>> > >
> >>> > > It has to handle this situation anyway to deal with holes
> >caused by
> >>> > > digest errors. This scenario occurs even with initiators that
> >issue
> >>> > > commands in order.
> >>> > >
> >>> > > >
> >>> > > > 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
> >>> > > > > >
> >>> > >
> >>> > > --
> >>> > > ##################################
> >>> > > Santosh Rao
> >>> > > Software Design Engineer,
> >>> > > HP-UX iSCSI Driver Team,
> >>> > > Hewlett Packard, Cupertino.
> >>> > > email : santoshr@cup.hp.com
> >>> > > Phone : 408-447-3751
> >>> > > ##################################
> >>> >
> >>> >
> >>> >
> >>> >
> >>>
> >>
> >
>