Friday afternoon hardware questions

[Bill McGonigle]

On Jan 23, 2008, at 22:05, Ben Scott wrote:

>   Perhaps not.  From what I've read, there is a standard way to
> connect multiple logical devices in a single SATA cable.  This
> facilitates a device called a "SATA port multiplier".  You have run a
> single (e)SATA cable from the host adapter to the port multiplier.

Right, that's SATA-PM.  The particular enclosure I linked to is just  
multilane, though.  Check out the rear picture - 3 infiniband  
connectors (3x4 for 12 drives).

Check out this unit:

   http://www.satadrives.com/4-port-sata-multilane-bracket-pci.html

and follow the copper traces to see how 'brilliant' the standard is.   
The good news is any drive and any controller can be tamed with a  
decent connector this way.

> So run one eSATA cable out of your computer and into the external
> enclosure, and put a port multiplier in the enclosure.

Or, even better, run SATA-PM over e.SATA Multilane and get that  
enclosure with 1 cable!  This becomes suddenly less silly.

>   This isn't a
> performance problem, because 3 gigabit/sec is still *way* faster than
> any physical disk can serve up.  Google found a good page:

That SATA 3Gbps spec is only 300MB/s after encoding, and a single  
drive can pop up to 120 these days if you're lucky.  Run an efficient  
RAID-10 over SATA-PM with a large data transfer and you're hitting a  
performance penalty with only 3 drives.  At least the silicon is  
getting a nice workout though.

Granted, many use cases aren't like that, and 4-ish drives would be  
just fine for many scenarios.

>   SATA really has far more in common with SCSI than the ATA of old.
> In effect, SCSI won the "IDE vs SCSI" war by marketing.  "People won't
> accept SCSI?  No problem; we'll just call it 'SATA' and everybody will
> be happy."  ;-)

Thank goodness.  And one of the SCSI standards, SAS, even uses the  
infiniband connector as a spec.  And most SAS controllers will run  
SATA drives.  So, yeah, huge difference. :)  AFAIK, SAS gets you: MAC  
addresses, longer cables, SCSI status commands (smarter than  
S.M.A.R.T).  But it's a bit more interrupt hungry than SATA, so there  
are wins both ways.

And just to be a bit on-topic, Linux calls everything SCSI these days  
anyway, even if it's IDE!

-Bill
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