Live ISO, VFS, and mount points

[startrail]

I am booting from an ISO for my OSdev and I've implemented the Joliet ISO filesystem.
Since the block device shows up as CD-ROM, the root filesystem also becomes read-only by my design.

I also don't have a separate VFS. I just select the appropriate FS as / and let each FS implementation switch to another FS whenever a mount point is encountered during path traversal.

Now, when I want to mount other filesystems at some mount point say /fs/<GUID>, I'm unable to create the directory /fs.
I was thinking of adding a flag isVirtual to my makeDirectory function which will essentially mean this is a memory-only directory entry and must not be committed to the underlying device. But that seems hackish to me.

I have 2 questions.
1. Is there a better way to create "virtual" directories?
2. How does Live Ubuntu ISO or any other live distro handle this? Assuming the same CD-ROM constraints.

[klange]

Is there a better way to create "virtual" directories?

Typically, you'd simply already have these directories present (but empty) on the read-only filesystem, so that they can be used as mount points for writable filesystems later.

How does Live Ubuntu ISO or any other live distro handle this? Assuming the same CD-ROM constraints.

A Linux live CD will typically use an overlay, putting an in-memory writable filesystem atop read-only data from a squashfs image on the CD.

Further, a Linux live CD may support an additional overlay system called casper, which allows changes to be written to (and read back from) a persistent storage device like a USB stick.

[AndrewAPrice]

Make empty placeholder directories on your disk drive if needed. Failing that, go with having an overlay.

I got around it by ditching the Unix hierarchy and not having anything mounted at /. My root directories are all virtual (mount points, shortcuts to installed applications, etc.)

[MollenOS]

I get around this by having a root fs mounted at '/' that is it's own filesystem, but a in-memory filesystem. It implements the same interface as a regular filesystem, except it has no backing device.

Then I just mount new filesystems under /storage/<dev>/<part>/.... and see if the the partition has a mount-point preferred by it's GUID and remount it under /... whatever.