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Date: Sat, 15 Nov 2014 18:08:38 -0800
From: Darren Pilgrim <list_freebsd@bluerosetech.com>
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To: Zaphod Beeblebrox <zbeeble@gmail.com>, 
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Subject: Re: OT-ish SATA port replicators vs. SAS "expanders"
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On 11/15/2014 2:42 PM, Zaphod Beeblebrox wrote:
> First... seriously... whoever was working on this naming convention thought
> "expanders" was a good name?
[...]
> What is the real difference?  Is it possible to hack the driver to support
> (obviously not booting from) SATA port replicators on the LSI?

Yes, the nomenclature isn't great.  In short:

An SAS expander is basically a multi-lane version of fibre channel 
without all that WWN nonsense.  You usually get two or four SAS lanes 
per port.  These are external with fancy cables.  SAS expanders are 
often simply labelled as "external SAS port".

A SATA port replicator (or port multiplier) is more like a USB hub: you 
plug the replicator into a host SATA port and it provides more than one 
SATA port for devices.  I've yet to see one without interoperability 
problems.  They also add a big risk: if one of the devices on a 
replicator has a problem, the host can't reset individual replicated 
ports--just the host port.