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    Re: iSCSI : Review Feedback on iSCSI Rev 06


    • To: IPS Reflector <ips@ece.cmu.edu>
    • Subject: Re: iSCSI : Review Feedback on iSCSI Rev 06
    • From: Santosh Rao <santoshr@cup.hp.com>
    • Date: Tue, 22 May 2001 18:49:40 -0700
    • Content-Type: multipart/mixed;boundary="------------12125BAB172DF21D905E5410"
    • Organization: Hewlett Packard, Cupertino.
    • References: <C1256A50.002C0BBD.00@d12mta02.de.ibm.com>
    • Sender: owner-ips@ece.cmu.edu

    Julian,
    
    Thanks for the clarification. A few additional queries below.
    
    Regards,
    Santosh
    
    -----------------------------------------------------------------------------------
    
    julian_satran@il.ibm.com wrote:
    
    > 1) Suggest re-naming the following in the interests of better
    > clarity :
    > 
    > a) Rename the ExpDataSN in SCSI Response PDU to EndDataSN as was used in
    > the previous version of the draft. (indicative of being the final Data
    > sequence
    > number.)
    > +++ Meaning has changed - 0 means none (it was fffffff before) +++
    
    Have the semantics of ExpDataSN in the SCSI Response PDU changed (?).
    From the description, it sounds like it is a count of the no of READ
    Data PDUs sent by the target for the command. 0 implies none were sent.
    (no data xfer). Hence, the name EndDataSN would be fitting since it
    really is the last DataSN the target used for this READ I/O.
    
    
    
    
    > 3) Why does the new draft limit the size of text commands and nop commands
    > to
    > 4096 bytes ? What is the rationale behind the magic number selection of
    > 4096 ?
    > 
    > Since DataPDULength is defined in units of 512 bytes, how does iSCSI deal
    > with
    > fragmentation caused due to the usage of a DataPDULength < 4K and an
    > attempt to
    > perform a 4K text , login or nop operation ?
    > 
    > I believe such scenarios were discussed in an earlier thread
    > titled "Fragmentation & Reassembly issues in iSCSI."
    > (http://ips.pdl.cs.cmu.edu/mail/msg03031.html) and the resolution was to
    > enforce a restriction that the size of a login, nop & text operation be
    > limited to DataPDULength (?).
    > 
    > +++ we decided to drop negotiating trivia. A fixed maximum is more
    > practical
    > and 4096 will no create inconvenience for a long time   +++
    
    
    > 5)  Section 2.8.3
    > "The data length of a text command or response SHOULD be less than
    >    4096 bytes."
    > 
    > suggest re-wording the above to :
    > "The data length of a login, nop & text command or response MUST be less
    > than
    >  DataPDULength. For the leading session login, the length of the login
    > command
    >  MUST be less than 512 bytes."
    > 
    > The above will ensure that login, text & nop operations will not result in
    > fragmentation.
    > 
    > +++ I have a hard time understanding the argument +++
    
    
    One basic question. Does DataPDULength govern the max length of SCSI
    Data PDUs only or all iSCSI PDUs ? This does'nt come across clearly from
    the definition. If this only governs SCSI Data PDUs, then, I'd suggest
    the following re-wrod to make it more clear in the definition on page
    129 (Appendix E) :
    
    Change :
    
    "Initiator and target negotiate the maximum data payload supported for 
       command or data PDUs in units of 512 bytes."
    
    to :
    "Initiator and target negotiate the maximum data payload supported for 
       SCSI command or data PDUs in units of 512 bytes."
    
    The above issues (3) & (5) are relevant if DataPDULength governs the
    length of all iSCSI PDUs.
    
    
    > 9) Section 2.18.1
    > 
    > "Target requests logout".
    > 
    > Suggest removal of the above option. A target that needs to logout would
    > use
    > either :
    > "Target will drop the connection"
    > or
    > "Target will drop all connections"
    > 
    > The "Target requests logout" also does not allow the target to specify
    > which
    > flavor of logout it wishes to receive. It is better to remove this option
    > and
    > use only a single scheme.
    > 
    > +++ The reason for introducing it was to allow a maintenance operation
    > (getting out a NIC card) to start
    > at the target. However only the initiator knows when the connects gets
    > quiet. This is why will have to keep this form. The other 2 are for
    > emergency handling +++
    
    Why is "target will drop connection" or "target will drop all
    connections" not sufficient for this purpose ? In this case, the target
    would send the async message, drop the connection/session and abort all
    outstanding I/O. The initiator on seeing the async message would
    clean-up all outstanding I/O on that session/connection and would
    attempt to re-login after LogoutLoginMinTime.
    begin:vcard 
    n:Rao;Santosh 
    tel;work:408-447-3751
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    org:Hewlett Packard, Cupertino.;SISL
    adr:;;19420, Homestead Road, M\S 43LN,	;Cupertino.;CA.;95014.;USA.
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    email;internet:santoshr@cup.hp.com
    title:Software Design Engineer
    x-mozilla-cpt:;21088
    fn:Santosh Rao
    end:vcard
    


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Last updated: Tue Sep 04 01:04:38 2001
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