Very slow booting and running and braindead OS's 4538
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Several concepts are getting confused together -- one is the "boot media", and one is "the root filesystem".
It is common for them to be the same, but that is not a requirement.
The "boot media" can go away - the "root filesystem" must not.
If the "boot media" and the "root filesytem" are the same device, then the OS will need the boot media to stay around forever.
An important concept to keep in mind: "filesystem" and "physical device" is not a one-to-one mapping.
That bears repeating.
A "filesystem" is a logical construction that does not necessarily correspond directly to a physical device.
A filesystem can be spread across several physical disks, for purposes of "size", "reliability" or "performance". Multiple filesystems (of potentially different types) can be built on one physical disk. Or both. Imagine a filesystem built in four stripes, all of which are on the same physical disk (NB this would probably be a terrible idea, high cost and low benefit).
One of the steps during the boot process is "mounting root", where root might be a virtual disk loaded from an image stored on the boot disk, or the boot disk might also have a valid filestructure on it.
One can imagine a degenerate case root filesystem that has NOTHING in it, except the pointers to all of the other filesystems that are currently mounted.
Well, you could do that in Unix, but it's generally not done - the disk(s) with the system data are generally the same disk...
At the other extreme, modern personal unices (e.g. Linux) are frequently configured with one enormous filesystem that has everything in it.
Yes. Now you got it :-). It was in our shop because of the way we had to have...
Most machines are configured with some happy medium - a smallish root filesystem with most of the namespace provided by other filesystems mounted during boot time.
The root filesystem cannot go away insomuch as at any point you may need to find the filesystem that has "-path-to-my-file" on it, and that requires starting at the root, and finding the filesystem that haspath on it, recurse.
-- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- Computer software consists of only two components: ones and zeros, in roughly equal proportions. All that is required is to sort them into the correct order.