Transparent integration of a new drive into the folder structure of a file server?

Question

Hallo,
I'm looking for solution to a problem, that seams to be simple, but obviously is not that easy to solve:
I have to integrate a second external RAID system in to our MacOS X 10.6.8 Server based file sharing server that is accessed from Mac and Windows clients via AFP and SMB. I want to move one or two main folders from the old RAID to the new one, but this physical change has to be invisible to the users, as I don't want to confuse anyone.
So my idea was to mover the Folders to the new drive, and create symbolic links at the old locations pointing to the new locations. This works very well for the Windows clients accessing the server via SMB, but does not work for the Macs on AFP.
So I tried to move one folder to the new drive and mount it at the old location. Again this works fine for the SMB but does not work for AFP, the mount point does not show up in a directory listing via AFP.
Do you have any other idea how I might integrate the new drive in transparent way into the old folder structure?
Thanks for your input
Florian
PS: I could use SMB on the Macs but for some reason I, whenever I try to log into the server from a Mac via SMB, user name and password are accepted, but then the Mac client displays a message saying that I don't have the right to access the share. The same share works using AFP.

Answer 1

Define 'invisible' please.
Do you mean you need to be able to do this live, while users are on the system? or just that you can shut the machine down, reconfigure it, and bring it back up with the new configuration, even though the shares look the same to the users?
I'm guessing the latter, but it's worth asking.
Ultimately the problem lies in the way the file sharing systems deal with multi-volume sharepoints, and it's not easy. If you think about it, say you have a 1TB array handling the main sharepoint and you want to substitute one of the directories in that sharepoint with a new, empty 4TB array.
When the user mounts the sharepoint they get a little status bar at the bottom of the window showing the available space... how much is that? Well, initially it would be however much of the 1TB volume is unused... except if they switch to the linked directory they now have 4TB available (or thereabouts)... so the amount of space available has changed even though, from the user's standpoint, they're still on the same share.
It isn't valid for the OS to report 4 TB as free because that isn't the case unless you're in this specific directory.
It also isn't valid for the OS to report 5TB free, even though there is that amount of space altogether.
It also isn't valid for the OS to report 1TB free because you could upload 4TB of data if you put it in the right place.
There are few solutions to this. Microsoft sort of addressed this with their DFS solution in Windows Server, but it's not trivial.
Unfortunately you can't just blow it off and ignore the issue.
Off hand there's only one thing I can think of that *might* work. if it doesn't then you're down to using multiple sharepoints on the server, with users mounting both disks simultaneously (which can be automated, for what it's worth).
The one thing to try is to statically mount the second RAID at the appropriate location so that, as far as the OS is concerned, it looks like just another directory even though it's on a different disk. You'd do this by editing /etc/fstab and adding a line like
UUID=AABBCCDD-79F7-33FF-BE85-41DFABE2E2BA /path/to/mount    hfs   rw
to /etc/fstab (this file may not currently exist, so just create it as root).
The first field is the UUID of the drive (which you can get via diskutil info)
The second field is the path where you want this drive to appear in the filesystem - i.e. somewhere in the path of your sharepoint. There must be an existing (empty) directory at this path when the disk mounts.
The third field identifies the disk as HFS
The fourth field marks the disk as read-write
Now when the system boots it should locate this disk and mount it on top of the existing RAID volume. If you cd to your sharepoint you should see the existing drive and if you cd from there into your mounted directory you should be looking at your new RAID.
Now, this all works fine (or, at least, should do) from a standard OS standpoint. The big question is whether the AFP and SMB daemons honor and support this kind of setup... there's one way to try, of course...
Now for testing purposes you could mount it at a dummy directory, just to see whether it's available to network clients. If it is then your next step would be to clone the data from the directory onto the new drive, then edit the fstab to mount the disk at the appropriate location.
Note also that the entire volume will replace the directory you mount over - that means you can't replace two (or more) directories with one volume, but you can, of course, partition or setup your RAID into multiple volumes and mount each individual volume over a specific directory.