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The server in question is DELL PowerEDGE SC1435. I am thinking about buying a LSI 1068e based card with two internal SFF-8087 ports. I intend to use the machine with external multiple hard disk enclosure as a ZFS based file server. I would connect the card with SFF-8088 bracket (assume the length of 0.5m). Another SFF-8088 bracket on a hard disk enclosure should fan out to 4x SATA ports with intention to be used with sata disks (assume the length of 0.5m). What would be the maximum recommended length of the external SFF-8088 to SFF-8088 cable?

I have found out that maximum length of sata cable should not exceed 1m. For esata the same length sould not be longer then 2m. And for external SAS cables it should not be more then 8m. Nowhere have I found if cable or drive influences the max length. If I use SAS card and SAS cables to be converted into SATA cable in last 0.5m of the length, what rule should I abide to?

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1 Answer 1

up vote 2 down vote accepted

This is actually a complicated electrical question dealing with capacitance, crosstalk, signal loss, and a number of other factors. That stuff makes my brain hurt though, so as a rule of thumb let's just say that passive cables should be limited to the shortest length, and should always be kept under the specification limits.

As a practical matter a cable slightly longer than the specification limits will probably still work, but it would be Bad And Wrong of you to attempt such a thing, and the engineer who wrote the specification will find you and kill you in your sleep.


So for a configuration of:
[Controller]<--SAS Cable--><--SATA Cable-->[SATA Drive]
or
[Controller]<--SAS-TO-SATA Cable-->[SATA Drive]
the two total cable length between the controller and drive should be limited to a maximum length of 1 meter (the SATA limit).
One meter is a REALLY FREAKN' LONG CABLE -- I would be hard pressed to find a cable routing that would exceed that in most servers/racks. If you're in that situation with external cabling in your rack the usual suggestion is to move a machine to reduce the distance.


When active components are involved (as they would be in your case) things are a little different.
The length of any passive cable is still limited, but active components (at least partially) reset the distance counter, so in a configuration like this:
[Controller]<--ESATA Cable-->[JBOD SAS Backplane]<==SAS Cables==>[SAS Disks]
the ESATA cable length is limited to 2 meters (the ESATA specification limit), and the cables betwee the JBOD's backplane and the SAS drives is limited to whatever the SAS limit is (I think it's 3 meters but don't quote me on that).

As a practical matter you can't just keep cabling things together for an infinitely long chain (even if you're sticking an active component in at each distance limit) - eventually the delays introduced by the cabling and the hardware would start to cause problems. Whether you hit other practical limits before that (like controller bandwidth saturation).

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