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

[FUJITA Tomonori]

On Fri, 19 Jun 2009 11:52:00 +0200
Arne Redlich <arne.redlich at googlemail.com> wrote:

> 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:

Ah, I didn't know that both specs define ISER.


> 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 ia 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."

OK, then I guess that iSER wants limit MaxBusrtLength to
RDMA_TRANSFER_SIZE to avoid breakage and set the default
MaxBusrtLength value to RDMA_TRANSFER_SIZE to avoid the performance
drop (with initiators that are not configured with large
MaxBusrtLength values). Right?


Thanks,
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