RE: iSCSI: Out of order commands, was current UNH Plugfest

["Rod Harrison" <rod.harrison@windriver.com>]

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
>