[stgt] [PATCH 14/15] Fix iSCSI Data-Out solicitation

[Arne Redlich]

Am Freitag, den 19.06.2009, 17:44 +0900 schrieb FUJITA Tomonori:
> On Fri, 19 Jun 2009 10:37:49 +0200
> Arne Redlich <arne.redlich at googlemail.com> wrote:
> 
> > Am Freitag, den 19.06.2009, 16:54 +0900 schrieb FUJITA Tomonori:
> > > On Wed, 17 Jun 2009 08:46:50 +0200
> > > Arne Redlich <arne.redlich at googlemail.com> wrote:
> > 
> > > > This wasn't an interpretation as the specs leave absolutely no room for
> > > > that, I was just rephrasing them. Here's the verbatim quotes:
> > > > 
> > > > RFC 3720 (iSCSI), 10.8.4 (Desired Data Transfer Length and Buffer
> > > > Offset):
> > > > "The Desired Data Transfer Length MUST NOT be 0 and MUST not exceed
> > > > MaxBurstLength."
> > > > 
> > > > RFC 5046 (iSER), 7.3.6 (Ready To Transfer (R2T)):
> > > > "4. [..] To transform the R2T PDU, the iSER layer at the target: [...]
> > > >  d. MUST use the Desired Data Transfer Length from the R2T PDU as the
> > > > RDMA Read Message Size of the RDMA Read Request Message."
> > > 
> > > In iSER case, the Desired Data Transfer Length MUST not exceed WHAT?
> > 
> > MaxBurstLength (or the limit incurred by the implementation, i.e. the
> > iSER mempool, depends on which is smaller).
> 
> Thanks,
> 
> Can you tell me where I can find the description about it in RFC 5046?
> 
> And can you also tell me where I can find the description about the
> length limitation of Data-in in RFC 5046 (your patch uses the max
> burst in iSER)?

See above for the section info. It's a consequence of _both_ specs:
an R2T must not request more than MaxBurstLength bytes, and since an R2T
is transformed into an RDMA read, the RDMA read must not request more
than MaxBurstLength bytes.

For Data-In. That's a bit less clear. Here's the reasoning that lead me
to version in the patch.

RFC 3720, 12.13 (MaxBurstLength):
"The initiator and target negotiate maximum SCSI data payload in bytes
in a Data-In or a solicited Data-Out iSCSI sequence.  A sequence
consists of one or more consecutive Data-In or Data-Out PDUs that end
with a Data-In or Data-Out PDU with the F bit set to one".

Data-Ins are translated to RDMA Writes and iSER obsoletes MaxRecvDSL,
it's replaced with Initiator- / TargetRecvDSL but only for _control_
type PDUs:

RFC 5046, 6.2 (MaxRecvDataSegmentLength):
"[...] Similarly, the target MUST consider the value of its local
MaxRecvDataSegmentLength (that it would have declared to the initiator)
as having the value of TargetRecvDataSegmentLength, and the value of the
remote MaxRecvDataSegmentLength (that would have been declared by the
initiator) as having the value of InitiatorRecvDataSegmentLength.

The MaxRecvDataSegmentLength key is applicable only for iSCSI
control-type PDUs."

RFC 5046, 7.3.5 (SCSI Data-In):
"2. It MUST generate and send an RDMA Write Message containing the read
data to the initiator [...]
c. It MUST use DataSegmentLength from the SCSI Data-in PDU to determine
the amount of data to be sent in the RDMA Write Message."


HTH,
Arne

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