what is SAN

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# 1  
Old 06-29-2004
Question what is SAN

Hello all,

I have looked at the entire posting that have SAN in it and I'm still fuzzy on how SAN works. I understand that every disk array can be access from any server that needs it, but is there software that is install or NFS mount type situation. One post stated that if your format command on Solaris you would be able to see the SAN drive. That to me means that it is some how mounting the disk.
# 2  
Old 06-29-2004
First think about disk arrays. A disk array might plug into a computer's scsi interface. The computer doesn't see the real disks. It sees make-believe disks provided by the array. You typically connect to a serial port on the disk array to program exactly what these disks look like. But to the computer, the disk seem like local disks. It can build a filesystem and mount it. Or it can use the raw disk space as with a database. You would usually mount the disk, but this is not absolutely required.

Next, instead of that scsi inteface, imagine a computer that talks to an attached disk array via fibre channel. We actually have several of these. You need fibre channel drivers on the computer. But no other special software.

Now unplug that fibre channel cable from the disk array and plug it into a SAN server. A SAN server is just a disk array on steroids....lots of steriods. Dozens of computers connect to it. You telnet into the SAN server to define what the make-believe disks look like but once again, to the computer they are just local disks.

SANs do have additional features beyond what a disk array can do to support clustering and backups, but to just get a handle on it, just think of it as a massive super disk array.
# 3  
Old 06-29-2004
I've always believed that there is a bit of confusion in terms that people use. I have always thought of NAS as Network Area STORAGE and SAN as Storage Area NETWORK. Anything that connects to this special network is a NAS device and it runs on a SAN. That SAN can be ethernet or fibre channel. However, I think that the common usage for most people is that SAN means fibre channel network and NAS means ethernet.

So by that definition a SAN is a network of fibre channel (a protocol) connected by either copper wires or fiber optic cables. Each device on this SAN connects through an HBA (hardware bus adapter) and 1 or more fibre channel switches. When the SAN is first installed every device on that SAN can see every other device such as a server, a tape drive, a disk drive, a tape library, disk array, etc. The SAN administrator will then set up paths to allow certain devices to "see" only certain devices and block paths to other devices. It is possible to have more than 1 computer using a certain device. This is sometimes used for clustering failover or pehaps a shared tape drive.

When you boot your computer it will see every device on the SAN that it has access to and it will look like a locally attached disk to your computer. There is no security for these devices other than that path. For example, if both a unix server and an NT server see the same drive the NT system will not know about the unix system and will claim it as its own and write its own header on that drive destroying the data for the unix system. Many people will refer to their disk array as their SAN but in reality the SAN is the network connecting the disk array. You may call the array a SAN device but not a SAN itself.

I may be slightly off on some of this and please correct me if I am wrong but this is always how I've thought of it.
# 4  
Old 06-29-2004
keelba, you have it all mixed up. SAN and NAS have nothing to do with each other except perhaps to be competitors.
# 5  
Old 06-29-2004
That is exactly my point. Most people have a common definition of the terms. Just like I hear many people say something like "Do you have OpenView?" I know that they usually mean OpenView Network Node Manager but OpenView is a suite of products and Network Node Manager is but one product in that suite. People have their common terminology and use it even though it is not quite accurate.

Network Appliance makes NAS devices that connect via ethernet. However, they also sell fibre channel HBAs for their Filers to allow direct attach via SAN. Conversely, there are all kinds of SAN disk arrays out there that attach via fibre channel but you can easily buy a NAS head and connect via ethernet and call it a NAS device.

This type of stuff really confuses people and that is why I offered two definitions and said that most people refer to SANs as their fibre channel network and the arrays on it. It is not fair to say they have nothing to do with each other because they do coexist with each other.
# 6  
Old 06-29-2004
Thnaks for your reply,

Perderabo, you stated a SAN server, is this a Unix machine or a special machine? You also stated that the need fiber driver. Is this on all machine that would be accessing the data. Thus if you were accessing from a windows machine you need fiber NIC card and same for Unix side? I'm sorry to be asking soo much, just trying to get things right.
# 7  
Old 06-29-2004
We only have one SAN server which is a EMC box with 15 TB of disks. It runs an OS called Intregrity. I don't believe that it is based on Unix. Our NAS servers are Netapp boxes and they run an OS called On-Tap which does have a slightly unix-like feel.

You can get a NAS "head" that connects to the SAN server. This "head" box would get disk space from the SAN and feed it out on the network via NFS. I'm told that we may do this and retire out Netapps.
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