[OpenAFS] local cell mounts, and vos dump/restore btw. cells

[Nickolai Zeldovich]

> if you have a volume in cell A that contains mountpoints for other
> volumes in cell A, and you do a vos dump of it, and then restore that
> dump as a volume in cell B, will the mountpoints in the new(cell B)
> volume still point to the other volumes in cell A, or will they now
> point to nonexistent volumes in cell B?

Mount points are stored similar to symlinks: they are just a string
that represents the volume they're mounting.  You can see that string
by using "fs lsm", e.g.:

  contents-vnder-pressvre~> fs lsm /afs/athena.mit.edu
  '/afs/athena.mit.edu' is a mount point for volume '#athena.mit.edu:root.cell'
  contents-vnder-pressvre~> fs lsm /afs/athena/user/k/o/kolya
  '/afs/athena/user/k/o/kolya' is a mount point for volume '#user.kolya'

In the first case, the mountpoint includes the cell name explicitly;
in the second case, it specifies no cell name, so it'll inherit the
cell from its parent volume that contains this mountpoint.  Observe
that mountpoints are stored just as they appear above (i.e. just the
string), and it should be easy to figure out what happens in different
cases when you dump/restore in different cells.

It sounds like you might have a use for the rarely-used linked cells
feature.  If you make two cells linked, things like volume lookup will
fail over to another cell if a volume is not found in one of the cells.

-- kolya