The first key object that we have with storage foundation is disk. Now, if you think about it from a server perspective, it might have a couple of internal disks. And then, there’s going to be some number of ones on the SAN. Maybe these two are from an AMC array. Maybe these two are from a Hitachi array. Volume Manager, when it starts up, identifies all those disks. So from a Volume Manager perspective, we would have six disks, two internal disks and four external disks. Once those disks are under VM control, then we can create what’s called a disk group. A disk group is just a collection of disks. The key differentiator on how you do that collection of disks is typically by application.
Let’s say we have a sales order application running on this machine. We’re going to take these four disks and put them into a disk group and maybe we’ll call it orders. That way it’s relative to the application. Now that we have disks in the disk group, we can start carving them up and using them for something.
The first thing we do is we create what’s called a volume. A volume is basically just a collection of disks for some purpose. For example, maybe we create a volume across those three disks and we call it ordersvol. Then maybe we create a volume on that disk and we call it customer for customer. Therefore, whatever the application needs, you use the disks in the disk group to create those volumes.
The next object is called the Plex. A Plex is within a volume and it’s a complete copy of the data. In this example with ordersvol, whatever is inside ordersvol (rather it is a file system or a database) a Plex is a complete copy of that data. When we have more than one Plex, we have more than one copy of the data, and this makes us have redundancy. I’ll come back to that in a minute though.
The last thing we have within a Plex is called the Subdisk. A subdisk is contiguous disk space on one of the disks that makes up that volume. All of these objects are actually just logical objects except for the subdisk. They’re the only thing that physically exists. Looking at it from a little different view, it would look like this. We have a volume, within a volume we have a Plex, within a Plex we have a single subdisk.
If I had a second Plex, maybe I would then have two subdisks. As I mentioned earlier, multiple Plexes gives you redundancy. Therefore each Plex is a complete copy of the data. So if I actually had a file system, maybe called orders, and I had three files in that file system, that complete file system is located within each Plex. So, another way to look at that, in this example Plex, this entire thing would be a single subdisk maybe on this first disk. That second Plex is made up of two subdisks. The first one maybe has the A file. The second one has the B and C files. All in all, though, this entire Plex is a complete copy of the data, one subdisk. And the other Plex is a complete copy of the data, two subdisks. And that’s the “basic” walk through of storage foundation objects.