RFC 3783 (rfc3783) - Page 2 of 14

Small Computer Systems Interface (SCSI) Command Ordering Considerations with iSCSI

RFC 3783                    Command Ordering                    May 2004


   4.  The Command Ordering Scenario. . . . . . . . . . . . . . . . .  8
       4.1.  SCSI Layer . . . . . . . . . . . . . . . . . . . . . . .  8
             4.1.1.  Command Reference Number (CRN) . . . . . . . . .  8
             4.1.2.  Task Attributes. . . . . . . . . . . . . . . . .  8
             4.1.3.  Auto Contingent Allegiance (ACA) . . . . . . . .  8
             4.1.4.  UA Interlock . . . . . . . . . . . . . . . . . .  9
       4.2.  iSCSI Layer. . . . . . . . . . . . . . . . . . . . . . .  9
   5.  Connection Failure Considerations. . . . . . . . . . . . . . .  9
   6.  Command Ordering System Considerations . . . . . . . . . . . . 10
   7.  Reservation Considerations . . . . . . . . . . . . . . . . . . 11
   8.  Security Considerations. . . . . . . . . . . . . . . . . . . . 12
   9.  References and Bibliography. . . . . . . . . . . . . . . . . . 12
       9.1.  Normative References.. . . . . . . . . . . . . . . . . . 12
       9.2.  Informative References . . . . . . . . . . . . . . . . . 12
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
   11. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 13
   12. Full Copyright Statement . . . . . . . . . . . . . . . . . . . 14

1.  Introduction

   iSCSI is a SCSI transport protocol ([iSCSI]) designed to enable
   running SCSI application protocols on TCP/IP networks, including
   potentially the Internet.  Given the size and scope of the Internet,
   iSCSI thus enables some exciting new SCSI applications.  Potential
   new application areas for exploiting iSCSI's value include the
   following:

      a) Larger (diameter) Storage Area Networks (SANs) than had been
         possible until now
      b) Asynchronous remote mirroring
      c) Remote tape vaulting

   Each of these applications takes advantage of the practically
   unlimited geographical distance that iSCSI enables between a SCSI
   initiator and a SCSI target.  In each of these cases, because of the
   long delays involved, there is a very high incentive for the
   initiator to stream SCSI commands back-to-back without waiting for
   the SCSI status of previous commands.  Command streaming may be
   employed primarily by two classes of applications - while one class
   may not particularly care about ordered command execution, the other
   class does rely on ordered command execution (i.e. there is an
   application-level dependency on the ordering among SCSI commands).
   As an example, cases b) and c) listed earlier clearly require ordered
   command execution.  A mirroring application does not want the writes
   to be committed out of order on the remote SCSI target, so as to






Chadalapaka & Elliott        Informational