Please explain the difference in SAN and NAS...?

Ok, I understand one is a Storage Area Network and one is Network Attached Storage. Why would someone pick one over the other from a business perspective? Is NAS easier to set up? Does it matter whether it's primarily a Unix environment or a Windows environment on the front end? For example, does CIFS or NFS make a difference? I've heard that SAN is on it's way out and NAS is taking over. True?

We're talking BIG systems here - data center stuff, not home computers. I'm a business type (with a tech background) trying to get my head around storage systems for large companies - 200Tb and up (some above a Pb!). I've picked up some books, but haven't been able to glean the advantages of one over the other, and under what circumstances one might choose one over the other. Thoughts?

I'm no expert on SAN, but here's my take --

They're not really used for the same thing. Rather, they CAN'T be used for the same thing... NAS is for sharing the whole disk space to many machines through the LAN. SAN is for sharing fast storage (disk array, tape library, etc) to mulitple servers on the back end. So to say that NAS replaces SAN is unlikely at best.

NAS just means network attached. So you put some drives in a device and attach it to the network. It becomes a file server on the network like any other, with all the limitations of any file server on your LAN (like the bandwidth bottleneck). The nice thing about using an appliance instead of a server for NAS is the lower configuration cost (in time, at least) and hopefully lower upkeep since there aren't a lot of Windows issues like updates, patches, corrupted files, blue screens, hardware glitches, etc. Generally, NAS on an appliance is reliable and consistent, and once it's set up, it just works. But it does not provide the robust disk access that servers need. Taking your data center example, think about a transaction server -- thousands of sales coming through and the database has to update all the info reliably and VERY FAST. Would you want to send it through the LAN? It just doesn't serve the same purpose as a SAN.

SAN, in my experience anyway, is used to share connectivity to a large hard drive array between one or more servers.

In the same way that you might like to access a big USB external drive from more than one box (how? you can't!), you might like in a data center to buy one enourmous RAID cabinet and connect more than one server to the drives. This is system level access, not file sharing access. It's at the 'hard drive' level that the servers see and use this connection. Hope that makes sense.

Let's say I need a big file share and also a big array for SQL server databases. I already have a server dedicated to each purpose, but the internal drives are too small on both. Let's say that space is limited, or the cabinets are expensive, whatever. So I get one big 8U cabinet full of hard drives. Without SAN, the only server that can take advantage of these drives is the one connected by the high-speed SCSI cable. Typical scenario for an external drive array is 1:1 server to cabinet. When a SAN is employed, other servers can use portions of the array. NOT the SAME portions, obviously, because you'd have all sorts of file locking problems and the speed would go way down. But the same cabinet of drives, divided up Logically -- some to one server, some to another. Now you can buy more storage in one large device and attach to it from different servers over a very fast Fiber Channel (hard drive speed) connection from additional servers, taking advantage of the economies of scale (more drives in one cabinet costs less than more drives in two or more cabinets). It also allows the flexibility of switching one server's access to the array while leaving another's alone.

Maybe a year from now, that SQL server is going strong, but the File server needs to be repurposed and a Web application server is employed instead. A simple cable swap to the Web app server is all that's needed to add him to the storage network. No messy hard drive swapping required.

Bottom line -- SAN will not be seen by the end users on the network. Nobody at their desktop "saves to the SAN" because it's transparent to them. It's a server resource, not a LAN resource. They would however see the NAS as a file share and write directly to it.

Hope that helps delineate what the two are for and why they're different.