Spread Spectrum Clocking

mtrupi

Gawd
Joined
Mar 26, 2007
Messages
739
Has anyone seen lower drive data transfer rates with SSC enabled for SATA or SAS drives?
 

UICompE02

SCSI Master
Joined
Jul 16, 2001
Messages
866
Theoretically, there would be a small performance penalty. SATA SSC works by varying the clock rate from the maximum supported speed down a very small amount, and then back up to the max (3.0gb/s). Realistically, the difference in the clock rates between the maximum (with no SSC) and the lowest rate it runs at with SSC enabled is such a small percentage of the available bandwidth that you'd be hard pressed to notice any performance difference between SSC and non-SSC behavior.

SATA SSC works by slowing the clock rate by 5,000 parts per million, over a 30Khz period. This leads to an average slowing of the clock by 0.25%, so that would be the expected performance difference between the two settings.

SAS is a little different with regard to SSC. SAS1 compliant devices (associated with a maximum speed of 3.0gb/s usually) do not support SSC. SAS2 devices support two methods of SSC. The first is Down-spread, like SATA uses, which is subject to the same performance hit as SATA. The second is Center-Spread, which varies the clock frequency both up and down from the target link rate, so there is no performance hit in that case.
 

UICompE02

SCSI Master
Joined
Jul 16, 2001
Messages
866
In a non-SSC configuration, all the EMI (Electro-magnetic interference) is radiated at a single frequency. Spread spectrum reduces EMI by varying the clock slightly to flatten out the frequency spike across a wider range.
 

mtrupi

Gawd
Joined
Mar 26, 2007
Messages
739
That was a very nicely done answer. Thank you. Have you seen basically no performance hit in real life use as well?
 
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