OT-ish SATA port replicators vs. SAS "expanders"

[John-Mark Gurney]

Brandon Vincent wrote this message on Sun, Nov 16, 2014 at 17:26 -0700:
> On Sun, Nov 16, 2014 at 4:51 PM, Mike Tancsa <mike at sentex.net> wrote:
> > I don think a port multiplier is the same as a SAS expander and a SAS/SATA
> > controller does not necessarily support port multipliers.
> 
> You're absolutely correct. The SATA specifications if I recall
> correctly, outline that providing support for port multipliers is
> optional. Not to mention, the vast majority of chipsets that do have
> support for port multipliers only provide the slower command-based
> switching method [1], The LSI SAS 2008 chipset is not what you
> typically find on the cheap consumer "fake RAID" cards which most
> people have [2].
> 
> Most people who purchase this card are purchasing it for SAS, true
> RAID, performance, and reliability. Thus, LSI is only going to provide
> support for the far superior SAS expander technology.

If you go w/ SATA PMs, make sure you get a controller that supports
FIS-base switching instead of command based switching...  You'll get
much better performance w/ NCQ support...   As for if the LSI SAS 2008
will support FBS, probably not, as it'll require the driver and/or
firmware to be aware of how to handle things...

This page has a list which includes if the controller supports FIS/FBS:
https://ata.wiki.kernel.org/index.php/SATA_hardware_features

It doesn't list the Marvell 88SE9235 that I'm using (but not w/ PMs),
but Marvell's site says it does, and it uses the ahci driver, which
means it should if you have a new enough version of FreeBSD that
supports PMs...

-- 
  John-Mark Gurney				Voice: +1 415 225 5579

     "All that I will do, has been done, All that I have, has not."