Re: SAS overview

[Jeff Garzik <jgarzik@pobox.com>]

Douglas Gilbert wrote:
> Scott M. Ferris wrote:
> 
>> Matthew Wilcox wrote:
>>
>>> On Tue, Mar 16, 2004 at 09:46:31PM +1000, Douglas Gilbert wrote:
>>>
>>>
>>>> Now things start to get interesting. SAS can aggregate
>>>> individual phy interconnects to form "wide" links. So
>>>> if two (single phy) cables are run from a HBA (occupying
>>>> two of its phys) to phys on the same expander then that
>>>> is a wide link. Seen from Linux driving that HBA that is
>>>> two PCI devices (each HBA phy) but only one SCSI initiator
>>>> port. Up to 256 phy interconnects can be ganged to make
>>>> a very wide SAS link (a parallel bus you might say).
>>>
>>>
>>> It's different though (and I suspect you know this, but let's clarify 
>>> for
>>> the audience).  A parallel link would send each bit down a different 
>>> phy.
>>> PCI-E (and probably SAS) send each byte down a different phy.
>>
>>
>>
>> The assumption that each phy will be a different PCI device is likely
>> to be false, at least for initiators.  
> 
> 
> Scott,
> Whether two phys on a SAS HBA are two separate SCSI initiators
> or one initiator on a wide link depends on what those phys are
> attached to. That seems pretty dynamic so it seems to me the HBA
> driver needs to look at phys individually. So if the phys are
> not separate PCI devices then they at least need to be individually
> addressable (and the SAS address isn't going to help since it's
> the same for all HBA phys). At the moment for SPI PCI HBAs sysfs
> has a one to one link between a PCI device and a SCSI host.
> Such sysfs links might look a little different in the presene of
> SAS wide links.

If it's anything like SATA, you have one phy per point-to-point link. 
Thus, 8 sata phys on a single 8-port SATA board.  Each is individually 
controlled and reset-able, and can throw transport error conditions 
independent of the other phys.

	Jeff