According to the Wikipedia page external SAS cables are specified for up to 10m length.
However, I found it pretty hard to actually find places that sell cables of that length. This made me wonder:
Are there any known problems with using cables that are as long as this? Will it be more fragile? Slower?
And if 10m is not suggested, would 6m be any more stable?
A little background: for several reasons we'd like to put a tape library physically separate from our main server and 10m would be enough to put it on a separate floor.
I haven't had any problems with 10m SAS cables, and if the ones you get are standards-compliant, (which they’re required to be, if they call themselves SAS cables) there wouldn't be. Speed or durability won't be impacted by cable length: the only issue you may run into would be signal strength or error rate, but as you're staying within the length specified by the standard, that shouldn't be an issue either.
The only thing you'll probably need to look out for is sources of interference - if the cables aren't shielded, running them near other unshielded cables or power sources might cause issues. This is true of any cabling, but obviously, the longer the cable, the more opportunities you have to run it by sources of interference.