How many SAS addresses should an expander has?
Elliott, Robert (Server Storage)
Elliott at hp.com
Wed Oct 16 09:31:27 PDT 2002
* From the T10 Reflector (t10 at t10.org), posted by:
* "Elliott, Robert (Server Storage)" <Elliott at hp.com>
> -----Original Message-----
> From: Seto, Pak-lung [mailto:pak-lung.seto at intel.com]
> Sent: Wednesday, October 16, 2002 9:37 AM
> To: 't10 at t10.org'
> Subject: How many SAS addresses should an expander has?
> * From the T10 Reflector (t10 at t10.org), posted by:
> * "Seto, Pak-lung" <pak-lung.seto at intel.com>
> First, ignoring the STP/SATA attachment first.
> For all links within an expander, during the Identify address
> frame phase, according to the SAS address field definition on
> page 127 v2.0a
> "An SAS ADDRESS field indicates the SAS address of the device
> transmitting the IDENTIFY address frame."
That wording is a bit dated. To be more precise it should say:
a) indicates the SAS address of the port for end devices; or
b) indicates the SAS address of the expander device for expander
> From the above definition, it means if any number of links
> from an expander are connected to an end devices, it will
> automatically (wants to) become a group of wide links and
> there is no choice of keeping it as individual separate
> Also, what happen if the SAS Addresses received from
> the end device(s) are not the same for all those links?
Each set of matching input addresses causes a separate expander port to
> In addition, in current SAS spec. individual links are
> performing link initializaiton/speed negotiation
> independently. And it is not guaranteed that all links
> within a wide links group to be negotiatied to have the same
> negotiated link rate. It may have problem if this wide links
> group happens to have the lowest negotiated link rate on the
> pathway between a particually initiator/target pair. I think
> the standard should either require all links within a wide
> links group to negotiated to have the same link rate (I
> believe it is a little bit to restrictive), or if any phy of
> a wide links group when it is reporting it's link rate during
> discovery to a initiator, it shall report the lowest
> negotiated link rate of the wide links group that the phy is
> belonging to (since the expander can select any link within a
> wide links group to make a connection), because the initiator
> has no concept of multiple minimum link rates for a pathway
> connecting to a particular target.
The expander never reports a link rate for the port; it reports
everything on a phy by phy basis. If a wide link consists of
physical links running at different rates, the initiator will know.
Assume there is a wide link with both 1.5 Gbps and 3.0 Gbps physical
links. If an initiator sends an OPEN (3.0) that needs to use that
wide link, that connection can only use a 3.0 Gbps physical link
of that wide link. If the initiator sends an OPEN (1.5), the
connection may use a 1.5 Gbps link or a 3.0 Gbps link (with rate
matching by the expanders).
Rob Elliott, elliott at hp.com
Industry Standard Server Storage Advanced Technology
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