Re: iSCSI: remove CDB from iSCSI header

[Matt Wakeley <matt_wakeley@agilent.com>]

julian_satran@il.ibm.com wrote:
 
> Matt,
> 
> Considering that most of the traffic is SCSI commands and responses the
> actual space waste is minor.

Assuming short reads/writes... If you are performing the large tape I/Os that
you like to refer to (your reason for having DataRN) then it's a larger
overhead.

> What you just suggest (based on misreading the accompanying text - I think)

I did not mis-read anything - perhaps you did:

Barry said:

B: So if the bidi-read length and ADDCDB fields are not in the data digest
B: then I assume we have to figure them into the header digest even though
B: they are located past the header digets. Is that the expected behavior?

Barry asked/stated if the bidi-read length and ADDCDB fields are past the
header digest to which you replied:

J: yes,
J:
J: Julo

> will force several reads for most of the traffic and we found this harmful
> to performance.

> Considering that most of the traffic has 48 byte headers the outlay we have
> now is optimal.
> The digest comes anyhow after the total header (including the additional
> fields).

As I stated earlier, that was not what your "yes" reply to Barry indicated.

> 
> Julo
> 
> Matt Wakeley <matt_wakeley@agilent.com> on 23/01/2001 23:16:17
> 
> Please respond to Matt Wakeley <matt_wakeley@agilent.com>
> 
> To:   ips@ece.cmu.edu
> cc:
> Subject:  iSCSI: remove CDB from iSCSI header
> 
> I don't think I like this.  Header followed by digest followed by more
> header.
> 
> I think this is a good opportunity to think about removing the CDB from the
> header.  This will reduce the amount of dead space used by headers that do
> not
> contain a CDB.  This will be beneficial when sending multiple smaller PDUs
> (in
> order to keep the CRC coverage high) by reducing the iSCSI header overhead.
> 
> Change the iSCSI command so that there is an iSCSI header (verified by the
> header digest) followed by CDB "payload" (verified by the data digest).  No
> options for immediate data.  Keep it simple, like it is in Fibre Channel.
> 
> -Matt Wakeley
> Agilent Technologies
> 
> julian_satran@il.ibm.com wrote:
> >
> > yes,
> >
> > Julo
> >
> > "Barry Reinhold" <bbrtrebia@mediaone.net> on 22/01/2001 17:49:29
> >
> > Please respond to "Barry Reinhold" <bbrtrebia@mediaone.net>
> >
> > To:   mbakke@cisco.com, Julian Satran/Haifa/IBM@IBMIL
> > cc:   ips@ece.cmu.edu
> > Subject:  RE: Coverage of Data Digest when using Header Digests
> >
> > Ok,
> >      So if the bidi-read length and ADDCDB fields are not in the data
> > digest
> > then I assume we have to figure them into the header digest even though
> > they
> > are located past the header digets. Is that the expected behavior?
> >
> > -----Original Message-----
> > From: mbakke@cisco.com [mailto:mbakke@cisco.com]
> > Sent: Monday, January 22, 2001 10:22 AM
> > To: julian_satran@il.ibm.com
> > Cc: ips@ece.cmu.edu; Barry Reinhold
> > Subject: Re: Coverage of Data Digest when using Header Digests
> >
> > Barry-
> >
> > In particular, the data digest covers only the SCSI Data part of an
> > iSCSI message; the header digest covers everything else.  This means
> > that in an 8k write, the data digest will cover only the 8k, and the
> > header digest will cover everything else.
> >
> > Hope this helps,
> >
> > Mark
> >
> > julian_satran@il.ibm.com wrote:
> > >
> > > Barry,
> > >
> > > Considering that one of the reasons to have a separate header and data
> > > digest was to enable data
> > > to carried through proxies, virtualizers etc. the current thinking is
> > that
> > > the data digest will cover only the data and the header (including
> > > extensions) will be covered by the header digest.
> > >
> > > Julo
> > >
> > > "Barry Reinhold" <bbrtrebia@mediaone.net> on 21/01/2001 21:50:04
> > >
> > > Please respond to "Barry Reinhold" <bbrtrebia@mediaone.net>
> > >
> > > To:   Julian Satran/Haifa/IBM@IBMIL
> > > cc:
> > > Subject:  Coverage of Data Digest when using Header Digests
> > >
> > > Julian,
> > >      Is the Data Digest intended to cover whatever follows the 48 byte
> > > iSCSI
> > > header? In particular in a command frame which has a CDB > 16 bytes,
> uses
> > > bidi, has immediate data, and is using both header and data digests -
> > what
> > > would the data digest cover?
> > >                                                              - barry
> > > reinhold
> >
> > --
> > Mark A. Bakke
> > Cisco Systems
> > mbakke@cisco.com
> > 763.398.1054