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    Re: iSCSI Requirements Draft - Informal WG Last Call



    
    
    There where many reasons for going to a total ordering.
    You will find them by looking up discussions starting with the Haifa
    meeting last summer.
    
    Julo
    
    Santosh Rao <santoshr@cup.hp.com> on 17/04/2001 21:06:56
    
    Please respond to Santosh Rao <santoshr@cup.hp.com>
    
    To:   Julian Satran/Haifa/IBM@IBMIL
    cc:   ips@ece.cmu.edu
    Subject:  Re: iSCSI Requirements Draft - Informal WG Last Call
    
    
    
    
    
    Julian,
    
    Ordered delivery can be achieved to better effect using the SAM-2 CRN
    based ordering due to the following reasons :
    
    1) CRN provides ordering on a per-lun basis and can be turned on and off
    for a subset of I/Os to that LUN. This allows for flexible ordering
    since ordering is a function of the I/O type from the application.
    Applications that are doing READ only operations (like a search engine)
    do not require any ordering. Ordering is required on metadata updates,
    any form of synchronization I/Os, WRITEs interspersed with READS, etc.
    Thus, an ordering solution should be flexible enough to be applied at
    the scope of a subset of I/Os destined to a LUN.
    
    Such an ordering scheme would also allow ordering to be turned on for
    only tape applications if disk applications did not require ordering.
    
    iSCSI's ordering solution does not provide this flexibility, whereas
    usage of CRN would.
    
    2) Such a fine granularity scope of ordering also minimizes the impact
    of error recovery actions taken when loss of order occurs. The impact
    with CRN would be a target-initiator handshake based on ACA + some
    checkpoints to error back all the pending CRN enabled commands on that
    LUN.
    
    Comparing the equivalent error recovery in a CmdSN based iSCSI ordering
    solution, the action taken would be to error back all the pending I/Os
    destined to the entire session following loss of order. With high end
    disk arrays having 1000+ LUN configurations, such error recovery is
    extreme, [especially when ordering may have been desired by the appln
    only on a small subset of I/Os to 1 LUN, and loss of ordering for the
    remaining 999 LUNs was a don't care].
    
    3) A CRN based ordering scheme works for all underlying SCSI transports
    as opposed to CmdSN based ordering.
    
    4) The generation of a stream of commands that expect strong ordering
    will need to be accompanied by corresponding generation of a sequence
    number at the same layer. (CRN would provide such a sequencing). Failure
    to do so can result in silent loss of order that slips un-detected due
    to potential points of failure in the stack b/n the SCSI ULP and the
    physical bus/link. (ex : I/O failures within the HBA driver due to
    resource allocation failures or other such conditions can cause loss of
    order.).
    
    Attempts to enforce ordering at multiple layers of the stack (CRN at the
    ULP and CmdSN at the LLP), especially when CmdSN does not provide all
    the benefits that CRN would provide is over-engineering the solution to
    the ordering problem. It also impacts iSCSI performance.
    
    - Santosh
    
    >
    > julian_satran@il.ibm.com wrote:
    > >
    > > Ordered delivery of commands to ANY TYPE of devices will increase in
    > > importance as network speeds increase and the need to hide latency
    > > increases.
    > >
    > > Today databases don't use queuing and rely and trickle the commands to
    > > devices 1 by 1 to ensure atomicity and order.
    > > As latency will become the determining factor in performance this is
    bound
    > > to change.
    > >
    > > SCSI has done an excellent job in defining the queueing mechanism. We
    have
    > > to make it work with good performance in our environment.
    > >
    > > Julo
    > >
    > > Santosh Rao <santoshr@cup.hp.com> on 13/04/2001 04:33:45
    > >
    > > Please respond to Santosh Rao <santoshr@cup.hp.com>
    > >
    > > To:   ips@ece.cmu.edu
    > > cc:   Black_David@emc.com
    > > Subject:  Re: iSCSI Requirements Draft - Informal WG Last Call
    > >
    > > David & All,
    > >
    > > I object to the following requirement :
    > >
    > > " MUST support ordered delivery of SCSI commands from the initiator to
    > > the
    > >   target, to support SCSI Task Queuing. "
    > >
    > > Ordered delivery is not a requirement for disk based applications and
    > > non tagged queueing tape applications, which form the majority of
    > > today's data traffic.
    > >
    > > To impose strict ordering (even in the presence of errors ?) as a MUST
    > > is penalizing the majority of today's data traffic that does not expect
    > > ordering from the SCSI subsystem.
    > >
    > > I am particularly concerned about the effect of the above requirement
    in
    > > the presence of errors. Does iSCSI expect strict ordering to be
    > > maintained even when individual I/O errors like ULP timeout occur ?
    > >
    > > On a ULP timeout (caused by, say, a hole in CmdSN), the initiator may
    > > choose not to retry the command, but instead, error it back to the ULP.
    > > In such a case, it can plug the hole in CmdSN with a NOP-OUT.
    > >
    > > The above requirement is not feasible to be met under such
    circumstances
    > > and others similar to this. Mandating strict ordering on ULP timeouts
    > > implies a session level error recovery on any individual I/O being
    > > failed back from iSCSI to SCSI ULP. This is a very heavy hammer to use
    > > as error recovery and should not be imposed.
    > >
    > > The above requirement must be changed to :
    > > " SHOULD support ordered delivery of SCSI commands from the initiator
    to
    > > the
    > >   target, to support SCSI Task Queuing. "
    > >
    > > - Santosh
    > >
    > > Black_David@emc.com wrote:
    > > >
    > > > It is intended to submit draft-ietf-ips-iscsi-reqmts-02.txt
    > > > as an Informational RFC. There is no formal requirement for
    > > > a WG Last Call, but if you have any further substantive comments
    > > > on the document please raise them on this list within the next
    > > > two weeks, i.e. by April 27th at the latest.
    > > >
    > > > If you have typographical/editorial comments please send them
    > > > direct to the document's author, Marjorie Krueger
    > > > <marjorie_krueger@hp.com>.
    > > >
    > > > Thanks,
    > > > --David and Elizabeth, IPS WG co-chairs
    > >  - santoshr.vcf
     - santoshr.vcf
    
    
    
    


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