iSCSI : Concerns regarding DataRN in READ Data PDU.

[Santosh Rao <santoshr@cup.hp.com>]

Julian/All,

Can anybody comment on the true benefit that the DataRN feature provides
that justifies the complexity and performance issues it introduces to
the protocol ?

I see the following issues with DataRN :

1) It adds a performance penalty in that on every READ Data PDU
targets will have to initialize one additional 32-bit word in the
header.

2) Extra traffic on the outbound context from initiators sending
NOP-OUTs to acknowledge the DataRNs.

3) Performance path checks for the "P" bit on every READ PDU.
(Alternatively, checks for the DataNumber field from the Login
Text Keys).

4)  Implementing DataRNs per task is too short-lived to provide any
real benefit. Implementing them per connection or session can result
in quick use up of the 32-bit DataRN space (since each frame consumes
one DataRN), adding additional complexity to the target code to use the
"P"
bit appropriately.

5) Further, "DataNumber", which is the size of the un-acknowledged
DataRN window is negotiated at login time, whereas this is really
desired to be a dynamic variable based on the target's current I/O load
and the number of initiators speaking to it. Thus, DataRN can end up
being
under-utilized resulting in too much NOP-OUT traffic, or it can be
over-utilized resulting in too much usage of the "P" bit in the READ
PDUs, which, again, can cause too much NOP-OUT traffic.

6) Considering that the DataRN generation and acknowledgement are
functions
that would typically be in SCSI hardware assist engines and there is a
need to
keep these as simple as possible, adding such features in the spec means

that these SCSI assist engines MUST implement DataRN generation
and acknowledgement [since initiators have no way to turn it off if a
target decides to use it]

What is the true benefit of this feature ? Is it intended to be used as
:

a) Will allow targets to do partial freeing of their memory buffers as
and when DataRNs are received ?

b) Will allow targets to send back only the un-acknowledged data (as
seen from the ExpDataRN received on the last NOP-OUT from the initiator)
when the initiators retry commands with the "Retry" bit set ?

If the benefit intended is (a), then, in order to derive this benefit,
targets will have to advertise their DataNumber login key to be < the
avg. number of frames transmitted per READ I/O. Having a DataNumber >
the avg. number of frames per READ I/O implies the life of the task ends
[and buffers for the task get released] before the initiator sends
NOP-OUT, thereby, defeating benefit (a).

If the benefit intended is (b) [and i do not see this explicitly stated
in the draft], then, this is a dangerous assumption which can lead to
data corruption issues.

When commands are retried with the "Retry" bit set, are targets allowed
to send back partial data based on the last ExpDataRN they received ? If
so, this can have multiple issues like :

o    complexity in handling I/O underrun cases for the retried command
      which only saw partial data transfer from the target.

o    If the target only sent back partial data based
      on its last ExpDataRN received, then, it can open up possible data

      corruption windows.

All in all, I believe that the DataRN is adding too much complexity and
potential performance impact to iSCSI for the benefits it may provide.

Moreover and most importantly, the draft does not give initiators any
control over the usage of this feature and if a target decides to use
this feature, initiators will have to support it. This exposes the
initiators to all of the above issues without being able to turn off
this feature.

I would like to therefore ask that :

either,
a) DataRN support be removed from the iSCSI draft.

or
b) The support for DataRN be negotiated at login time giving initators
control over this feature and allowing them to disable this feature.
This will allow SCSI hardware assist engines to skip implementation of
this functionality and their associated software can merely turn off
DataNumber in the login negotiation.

Thanks,
Santosh



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