RE: iSCSI: Out of order commands

["Robert D. Russell" <rdr@mars.iol.unh.edu>]

Somesh:

> > It seems to me that if a target can only handle x new commands,
> > then its queueing capacity is x, and in the SCSI Response PDU it
> > should set MaxCmdSN = (ExpCmdSn + x - 1), in accordance with the
> > formula in section 2.2.2.1.  This in turn controls the number of
> > commands the initiator can send, and thus prevents the incoming
> > commands from overfilling the target's queue.
> 
>   This is actually a weakness of the multiple connections over
>   multiple adapters model (it is not related to ordering).
>   The target advertises a command window on a session wide basis.
>   At the same time, it has to provide the resource to the adapter
>   to be able to DMA those commands to. Since there is no restriction
>   on which connection the commands may be received, either the
>   target has to allocate the max resources needed to each adapter
>   (thus committing n times the resources actually required), or
>   has to "lie" (it would not be a complete lie) which could
>   result in running out of resources. One way to fix that would be
>   to have the credit on a per connection basis.

If I understand you correctly, you are saying that deadlock can
occur even if we enforce ordered delivery by initiators and even
if the advertisements are correct and even if both parties obey
the advertisements.

In other words, doesn't this mean that the standard can lead to a
trap in implementing targets and that, in fact, the advertisements
really should be on a per connection basis?
However, what would be the consequences for initiators if
the advertisements were on a per-connection basis?


Thanks,

Bob Russell
InterOperability Lab
University of New Hampshire
rdr@iol.unh.edu
603-862-3774