SAS 2 question

Elliott, Robert (Server Storage) Elliott at hp.com
Tue Sep 30 16:40:07 PDT 2008


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When there are multiple connections between an initiator and target, STP and
SMP don't allow multiple their frames to wander across connections in the
same way that SSP allows the frames for one I_T_L_Q nexus to wander.
An STP transport layer assumes it is talking to only one STP link layer.  The
port layer (state PL_PM3) bridges the STP transport layer directly to the STP
link layer (those state machines are as defined in SATA).  The STP link layer
can change at times (after SATA_SYNC/SATA_SYNC, if the connection is closed),
but there's only one STP link layer at a time that is associated with a given
STP transport layer.  STP only allows one connection at a time between the
same initiator port and target port (see 7.18.5), since that's all an
STP/SATA bridge could reasonably implement.
An SMP transport layer assumes it is talking to only one SMP link layer.  The
state machines are defined in SAS, so they do use the same Transmit Frame
requests and Frame Received confirmations that SSP uses.  PL_OC2's frame
transmission section discusses the 1:1 relationship.  SMP does allow multiple
connections at a time between the same initiator port and target port.	Most
controller SMP initiator ports and expander SMP target ports only support one
at a time, though - management functionality is error-prone and usually not a
performance concern.
The SSP transport layer assumes the port layer may be scattering its traffic
across multiple SSP link layers (i.e., multiple connections).  Message
arguments like Request Fence exist to handle that.
STP and SMP ports can certainly be wide ports with multiple connections open
at a time to different destinations, though.  That just means they employ
multiple transport layers.
---
Rob Elliott, HP Industry Standard Server Storage
________________________________
From: Bill.Martin at Emulex.Com [mailto:Bill.Martin at Emulex.Com]
Sent: Tuesday, September 30, 2008 5:43 PM
To: Elliott, Robert (Server Storage); t10 at t10.org
Subject: SAS 2 question
In SAS2r14e, figure 28, it is implied that there is only one SMP or STP port
per physical SAS phy.  Is this intentional, or is it intended that there
could be one STP port per logical phy (as implied in figure 38)? I cannot
find any indication of what is done with a second SMP or STP request coming
in to the SAS phy on a second logical link.  Should this be covered, and if
so, where?
Thanks,
Bill Martin
Emulex
Office of Technology
Industry Standards
916 772-3658
916 765-6875 (Cell)
bill.martin at emulex.com



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