SAS 2 Muxing and DISCOVER

Craig Stoops craig at
Wed Mar 14 20:11:20 PDT 2007

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Rob, Steve, Brian and others involved:
I'm wondering about SMP Discover and interaction with Muxing. I've read the
recent proposal 07-091r0 regarding the Discover extensions, but I feel there
are some issues at hand. 
>From a higher perspective, there isn't much difference between 2 logical
phys with muxing and 2 physical phys. Each logical phy sends and receives an
Identify Frame (IAF). As such, each logical phy is free to advertise any
capabilities it sees fit, including a unique SAS address. The two logical
phys need not send the same information. The net result is, that the 2
related logical phys may either be viewed as 2 logical ports, or two phys as
part of a wide port. I know that when Muxing was suggested that being seen
as a wide port is what was in mind, but I can think of several target
devices that would like to advertize two different SAS addresses on the
muxed link. There is nothing I can find in the standard to prevent this, and
modeling show it works very nicely.
To the higher level layers such as port layer of Expander Function, there is
no difference between a muxed link and a non-muxed link. Only the SAS
addresses on that link determine whether the logical phy is part of a wide
port or not.
However, this implies that when addressing a SAS device that has muxing
enabled, that it should report the number of phy as as the number of logical
phys, not physical. And also, it needs to be possible to issue a Discover to
each logical phy, not just physical phy inorder to gain access to the
individual IAF results.
Another potential solution is to issue Discover for Phyical phys, but have
space to report the capabilities of each logical phy that is part of that
phycial phy, eg two attached sas address, two sets of capbiltiies (both
local and remote), etc
Craig Stoops
ExpertIO, Inc.
"Your Storage Verification and Design Experts"

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