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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] RE: Command Queue Depth (was asymmetric/Symmetric)Dear colleagues, Although the windowing mechanism in iSCSI-01 may seem to be there to solve a queueing issue it is mainly meant to limit the buffering space for commands that await "de-skewing". We assume that execution queue-lengths, policy etc. are beyond the scope of transport. As for SCSI queue length I assumed that the busy or queue full status followed by an Asynch Event message indicating readiness is the mechanism provided by SCSI to regulate the command flow. It is hard to imagine that give the variable life-time of SCSI commands and the opaque nature of the resources required to execute them that the transport has to help in this area. Julo Jim McGrath <Jim.McGrath@quantum.com> on 07/09/2000 06:06:40 Please respond to Jim McGrath <Jim.McGrath@quantum.com> To: "'Matt Wakeley'" <matt_wakeley@agilent.com>, ips <ips@ece.cmu.edu> cc: (bcc: Julian Satran/Haifa/IBM) Subject: RE: Command Queue Depth (was asymmetric/Symmetric) The issue of buffer space allocation for multiple initiators has a long and troubled history in SCSI. We have never been able to come up with a good answer. Fibre Channel tried to fix this with the notion of "login BB credit" - when you login you get a minimum number of credits you are always guaranteed when you start data transfers. The problem with this is that storage devices had no realistic ability to discriminate between initiators or to change the login BB credit. In addition, the expectation is that all possible initiators would get these credits on login. So storage devices vendors have played it safe and kept this number low (at 0 until recently, now around 2). For iSCSI the number of initial credits you need to "prime the pump" until normal data flow is established is probably large (given the latencies are higher than in Fibre Channel, especially FC-AL), and the number of potential initiators larger than in Fibre Channel, making this a whole lot worse for the storage device. As soon as we allow the devices to start adjusting these credits, then you have the protocol problem of making sure people know when their credits are adjusted and the policy problem of how, who, and when to adjust the credits. Changing everyone's credit when you add a new initiator can get into a notification nightmare, although it is "fair." Any policy brings up all sorts of nasty issues regarding fairness vs efficient use of the transmission media. Jim Note: the same problem has plagued other attempts to allocate device resources between multiple initiators, like command queue space. In general policies with respect to multiple initiators are not really standard in the SCSI world. -----Original Message----- From: Matt Wakeley [mailto:matt_wakeley@agilent.com] Sent: Wednesday, September 06, 2000 2:56 PM To: ips Subject: Re: Command Queue Depth (was asymmetric/Symmetric) Joshua Tseng wrote: > James, > > I agree with others that there may be an issue with the > command windowing mechanism in the existing iSCSI spec. It is like > "TCP in reverse", in that the target determines the size of the window, and > not the initiator as in TCP. Rather, I believe that everything that this > windowing mechanism is attempting to achieve can be more easily obtained > by having the target communicate its buffer size to the initiator at > iSCSI login. It should be the role of the initiator to determine how > many commands to put in flight simultaneously, given this input on available > buffer size from the target. As more initiators connect to a target, it may need to scale back the amount of this buffering it has allocated to each previously logged in initiator (to prevent rejecting new logins). > > > As far as multiple initiators, could this not be resolved by the target > refusing additional logins beyond the number of initiators it can safely > support? Not being a storage expert, this is my best guess/suggestion > at how to do it. I believe John already answered this... -Matt
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