practical maximum number of drives

[Graham Allan]

This may well be a question with no real answer but since we're speccing 
out a new ZFS-based storage system, I've been asked what the maximum 
number of drives it can support would be (for a hypothetical expansion 
option). While there are some obvious limits such as SAS addressing, I 
assume there must be more fundamental ones in the kernel or drivers, and 
the practical limits will be very different from the hypothetical ones.

So far the largest system we've built is using three 45-drive chassis on 
one SAS2008 (mps) controller, so 135 drives total. Over many months of 
running we had several drives fail and be replaced, and eventually the 
OS (9.1) failed to assign new da devices. It was time to patch the 
system and reboot anyway, which solved it, but we did wonder if we were 
running into some kind of limit around 150 drives - though I don't see why.

Interestingly we initially built this system with each drive chassis on 
its own SAS2008 HBA, but it ultimately behaved better daisy-chained with 
only one. I think I saw a hint somewhere this could be to do with 
interrupt sharing...

Thanks for any insights,

Graham