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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] RE: iSCSI Reqts: In-Order DeliveryHi: One minor comment: > (1) MUST provide ordered delivery of SCSI commands from > the initiator to the target in the absence of transport > errors visible to iSCSI (e.g., iSCSI CRC failure, > unexpected TCP connection closure). Does the term "SCSI commands" include task management functions as well? If not, it should. Charles > -----Original Message----- > From: Black_David@emc.com [mailto:Black_David@emc.com] > Sent: Friday, April 20, 2001 7:11 AM > To: ips@ece.cmu.edu > Subject: iSCSI Reqts: In-Order Delivery > > > About a week ago, Santosh Rao wrote: > > > I object to the following requirement : > > > > " MUST support ordered delivery of SCSI commands from the > initiator to > > the target, to support SCSI Task Queuing. " > > In the hopes of driving this discussion to closure, > let me summarize my understanding of this situation: > > - SAM2 expects ordered delivery, but does not mandate > it. Section 4.6.2 of SAM2r16 says: > > For convenience, the SCSI architecture model assumes > in-order delivery > to be a property of the service delivery subsystem. This > assumption is > made to simplify the description of behavior and does not > constitute a > requirement. > > I will observe that there is well-known code for disk > initiators (e.g., staircase/elevator/etc. command queue > reordering for seek optimization) that relies on this > expectation for performance (not correctness). IMHO, > making in-order delivery purely OPTIONAL would not be > consistent with the spirit/intent of the above SAM2 text. > > - In existing SCSI transports, transport errors (e.g., CRC failure) > can cause ordering issues at higher levels. Both Parallel SCSI > and Fibre Channel initially drop anything that has a transport > error. IF the higher level code retries AND other commands were > executed before the error was noticed by the retry logic, THEN > the retry occurs in a different place in the command sequence > from the original command. I believe that existing tape > implementations usually either don't retry, or spoon-feed the > tape one command at the time to make sure no other command can > get in the way. FCP-2 has the ability to prevent other commands > from being executed ... > > - iSCSI has to face this issue because we're incorporating a retry > mechanism into the base protocol. Parallel SCSI and FCP-1 could > ignore this because they didn't do retries. FCP-2 has a retry > mechanism, but makes order preservation of retries negotiable - > order of command delivery in the face of retries is only assured > if CRN is in use, and that is controlled by a settable bit in a > mode page (FCP2r7, 4.3). The Initiator can choose not to set > this bit, and the Target can refuse to allow the > Initiator to set > it. > > - CRN is a relatively new mechanism, and both it and the associated > FCP-2 retry mechanism are likely to be implemented for/used by > tapes. Disks will still have an expectation of > in-order delivery > in some cases, even though neither of these mechanisms > are in use. > IMHO, Santosh's proposal to use CRN for all > expectations/requirements > of ordering doesn't seem like a good idea for this reason. > > So, let me try the following proposal to resolve this issue/objection: > > (1) MUST provide ordered delivery of SCSI commands from > the initiator to the target in the absence of transport > errors visible to iSCSI (e.g., iSCSI CRC failure, > unexpected TCP connection closure). > (2) MUST specify the ability to preserve ordered delivery > of SCSI commands even in the presence of transport > errors. A mechanism MUST be provided to allow > Initiators and Targets to negotiate this preservation > on a per-session or finer granularity basis, > > The Rationale for (1) is the combination of the SAM2 expectation > plus the fact that there are situations in which disks expect > this ordering in the absence of mechanisms like CRN that can > enforce it. The Rationale for (2) is to provide support > analogous to FCP-2 - this should be sufficient for tapes to > obtain the behavior they require. > > Comments? > > Thanks, > --David > > --------------------------------------------------- > David L. Black, Senior Technologist > EMC Corporation, 42 South St., Hopkinton, MA 01748 > +1 (508) 435-1000 x75140 FAX: +1 (508) 497-8500 > black_david@emc.com Mobile: +1 (978) 394-7754 > --------------------------------------------------- >
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