* Which technology is right for you?
Today the Serial Attached SCSI expanders inside those JBODS or RAID systems allow you to extend the SCSI bus outside the boxes, daisy chaining them to hundreds of additional storage devices over an area of 100 meters. Moreover, put a SAS host bus adapter in an application server at both ends of the daisy chain and you have a data pool accessible to two machines.
It may well be that SAS daisy chaining will give rise to environments that use only a few large storage arrays; buyers can instead opt for a pay-as-you-go approach, adding a single new device module when needed – and, as your friends in accounting will tell you, paying for it in cheaper dollars.
But wait, there’s more! The arrival of SAS switches makes it feasible to route multiple servers to multiple independent JBOD or RAID systems. Inject a SAS switch into the mix (between the applications and the storage devices) and the number of application servers increases by the port count of the switch. How much storage you put on the other side of the switch is, as a matter of practicality, pretty much a function of how much money you want to invest.
With the sharing and pooling of large amounts of storage across a switch, all this is starting to sound like a “SAS fabric,” which is of course exactly what it is. Can we now talk about a “SAS SAN”? If you can deal with a term made up only of acronyms, why the heck not!
Now that SAS gets added to the mix, the list of choices for a SAN fabric just got a bit richer. But which of the available technologies is the right one for you? Here’s some preliminary guidance:
* How much connectivity do you need? SAS can handle 100+ nodes, Fibre Channel does 1,000+, and iSCSI does 100,000+.
* How much distance, end-to-end, is necessary? SAS can cover 100 meters, Fibre Channel does 10 kilometers, and iSCSI, well … how big is the Internet?
* How much throughput do you require? SAS does 12GB, Fibre Channel does 4GB, and iSCSI does 1GB.
* How price sensitive are you? In order of anticipated expense, it is very likely that iSCSI will be cheapest, Fibre Channel will be most expensive, and SAS will fall in between the two, but likely be closer to Fibre Channel (at least in the beginning).
What other CAPEX should you expect? With iSCSI, the added packet traffic may require an infrastructure upgrade to maintain adequate performance. With Fibre Channel, laying out the initial supportive infrastructure (HBAs, switches, cabling) will be a non-trivial cost. With SAS, it is hard to tell at this point, but most likely, a pay-as-you-grow approach will be the standard.
The market won’t stand still, of course, and you should expect to see things get even more interesting as the SAS, iSCSI and Fibre Channel technologies continue to mature. As a result, who knows which bus you may be riding into the future?
