SAS-2 Spread spectrum clock

Ralph Weber roweber at ieee.org
Wed Jul 5 06:42:26 PDT 2006


* From the T10 Reflector (t10 at t10.org), posted by:
* Ralph Weber <roweber at ieee.org>
*
To be honest, I do not fully follow everything being said in
this thread, but one suggestion sticks out like a sore thumb.
"Why don't we switch off the SSC of the SAS device, if it is
connected to a SAS-1.1 expander?"
Fibre Channel includes features where valid physical connections
are automatically disabled because of protocol interoperability
problems.
These features have generated deep anger in the user community.
They are a black stain in Fibre Channel products for some very
critical customers.
"Switching off" anything that is correctly connected is a very
bad idea. This is not a path to be followed without grave
consideration.
Rob, you will have to twist my are really hard to really hard
to get my vote for this kind of proposal and I suspect you
prefer things that way.
All the best,
.Ralph
Massimo POZZONI wrote:
>* From the T10 Reflector (t10 at t10.org), posted by:
>* Massimo POZZONI <massimo.pozzoni at st.com>
>*
>
>The SAS-II device could transmit to a SAS-II expander using +/-2500ppm.
>In this case we have the same SSC for TX and RX and the RX is not
>stressed.
>
>  
>
>>SAS-1.1 expanders/HBAs are supposed to support receiving +0/-5000 ppm
>>(I hope they were more compliant than SATA HBAs), but were not
>>expected to tolerate +2500/-2500 ppm.  SAS disk drive vendors would
>>prefer to implement just one behavior regardless of the expander/HBA
>>to which they are attached, so transmitting +0/-5000 ppm is the
>>only safe choice.
>>    
>>
>
>Why don't we switch off the SSC of the SAS device, if it is connected 
>to a SAS-1.1 expander? 
>As you mention, the SAS-1.1 expander is not supposed to receive
>SSC from a SAS device.
>
>  
>
>>Disk drive vendors would prefer not to include separate SSC controls
>>per phy.  Since there are generally more disk drives than HBAs or
>>expanders, it makes sense to keep them as simple as possible.
>>    
>>
>
>I completely agree on this request. Switching off the SSC in 
>case separate SSC controls would be required, can always solve this 
>issue.
>
>Massimo
>
>
>-------------------------------------------------------------------
>
>Date: Tue, 4 Jul 2006 15:04:22 -0500
>From: "Elliott, Robert (Server Storage)" <Elliott at hp.com> Add To Address
>Book
>Subject: RE: SAS-2 Spread spectrum clock -
>To: <t10 at t10.org>
>
>  
>
>>-----Original Message-----
>>From: owner-t10 at t10.org [mailto:owner-t10 at t10.org] On Behalf
>>Of Massimo POZZONI
>>Sent: Tuesday, July 04, 2006 2:13 AM
>>To: t10 at t10.org
>>Subject: SAS-2 Spread spectrum clock -
>>
>>* From the T10 Reflector (t10 at t10.org), posted by:
>>* Massimo POZZONI <massimo.pozzoni at st.com>
>>*
>>The 06-263r2 (SAS-2 Spread spectrum clock) specifies that:
>>
>>  - the device trasmitter SSC is 0/-5100 ppm (both SAS and SAS)
>>
>>  On the other hand,
>>  - expander transmitter is	 0/-5100 ppm if talking to a SATA device
>>  - expander transmitter is + 2500/-2500 ppm if talking to a
>>SAS device
>>
>>  Let's consider the receiver Clock Recovery (CDR) of the device.
>>  Let's assume that the receiver Clock Recovery makes use of the same
>>  clock reference used by its transmitter (to save power and area
>>  in the PHY).
>>
>>  As a consequence, this clock reference, used by the receiver CDR
>>  to recover the incoming data, will be SSC modulated as the
>>  transmitter.
>>
>>  We see now that the receiver CDR is much more stressed than in SATA.
>>  In fact its reference clock can be at -5000 ppm, while the data
>>  it is receiving can be at +2500 ppm. Total 7500 ppm.
>>
>>  My concern is for the Clock Recovery. It has been proven that
>>  many problems are already existing in sata devices, where the
>>  clock recovery is stressed up to 5000 ppm.
>>    
>>
>
>What problems?  I haven't seen any errata to the SATA specification
>trying to reduce the ppm range.
>
>There are many accounts on the Internet about problems arising from
>HBAs not supporting SSC as required.  Disk drive vendors that tried
>turning on SSC have had to provide consumers special programs or
>jumpers to turn SSC off so they will work in non-compliant systems.
>http://www.intel.com/support/motherboards/server/sb/CS-021230.htm
>http://www.xlr8yourmac.com/archives/nov05/111105.html
>http://discussions.apple.com/message.jspa?messageID=2379652
>http://www.barefeats.com/hard49.html
>http://www.seagate.com/support/kb/disc/faq/ssc.html
>http://www.wdc.com/en/library/eide/2579-001037.pdf
>http://www.hitachigst.com/hdd/support/download.htm
>
>  
>
>>  Now we are proposing to stress it at 7500 ppm !!!.
>>
>>  My proposal:
>>  If an expander is transmitting to a SATA device: TX @ 0/-5000
>>  If an expander is transmitting to a SAS  device: TX @ +2500/-2500
>>  (This is not changed)
>>
>>  If a SAS device is transmitting to a SAS expander: TX @ +2500/-2500
>>  (This is the new !!)
>>  If a SATA device is transmitting to a SAS expander: TX @ 0/-5000 ppm
>>    
>>
>
>SAS-1.1 expanders/HBAs are supposed to support receiving +0/-5000 ppm
>(I hope they were more compliant than SATA HBAs), but were not
>expected to tolerate +2500/-2500 ppm.	SAS disk drive vendors would
>prefer to implement just one behavior regardless of the expander/HBA
>to which they are attached, so transmitting +0/-5000 ppm is the
>only safe choice.
>
>HBAs would like to have as few choices (e.g. clock trees) as possible.
>They must use +0/-0 when attached to a SAS-1.1 phy, and can only
>use +0/-5000 ppm when attached to a SATA phy, so +2500/-2500 ppm
>would require a third clock tree that is not really necessary.
>Nothing should break if they did transmit +2500/-2500 ppm to a
>SAS-2 phy, since all SAS-2 receivers will be required to
>tolerate it.
>
>  
>
>>  If the SAS expander cannot tolerate the +2500/-2500 from the
>>   SAS device, the SSC is turned off in all the ports of the device.
>>    
>>
>
>Disk drive vendors would prefer not to include separate SSC controls
>per phy.  Since there are generally more disk drives than HBAs or
>expanders, it makes sense to keep them as simple as possible.
>
>  
>
>>   In summary: is it possible to have the same SSC range for both the
>>   transmitter and the receiver of a SAS device?
>>    
>>
>
>I don't think so.
>
>  
>
>>Massimo Pozzoni
>>    
>>
>
>--
>Rob Elliott, elliott at hp.com
>Hewlett-Packard Industry Standard Server Storage Advanced Technology
>https://ecardfile.com/id/RobElliott
>
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