Re: iSCSI: Out of order commands

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

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
> ##################################