Filesystem overlay ?

[William Park]

On Fri, Feb 04, 2005 at 05:13:38PM -0500, Dave Stubbs wrote:
> William Park wrote:
> 
> >I would appreciate if someone with better memory can kick me in the
> >right direction... 
> >
> >Suppose you have read-only filesystem (eg. CD-ROM) mounted on /var.  Is
> >there some overlay feature that you can enable (in kernel or
> >filesystem), so that /var appears to be read-write while the system is
> >on.  But, when you reboot, you're back to original /var content.
> >
> >I've remember something like this is possible in 2.6.  But, name or
> >keyword escapes me.
> >
> > 
> >
> Hi William,
> 
> Well, there's a few:
>  - unionfs - http://www.fsl.cs.sunysb.edu/project-unionfs.html
>  - ovlfs - http://ovlfs.sourceforge.net/ (this one appears to be dead, 
> but slax uses it)
>  - translucency - http://sourceforge.net/projects/translucency/
>  - mini_fo - can't find homepage, but Morphix seems to run on this
>  - cowloop - http://www.atconsultancy.nl/cowloop/
> 
> Apparently there are even more.  I found these by lurking in the slax 
> forums, watching threads between the Slax guy and people with all kinds 
> of suggestions to replace ovlfs.  Slax is a bootable CD Linux Distro 
> based on Slackware - you'd like it :-)  Apparently even lvm could be 
> made to do this, but that might be quite complex.

'ovlfs' rings a bell, but I don't think that was it.  Anyhow, thanks for
the URLs.  I'll check them out.

For completeness and for educational purpose, there are 2 standard ways:
    - Use 'initrd'.  Let it fall through the boot process, and mount it
      on /var.
    - Use 'tmpfs'.  Right after kernel mounts /var, populate it with
      stuffs from /clean-var.
I'm currently using 'initrd' because it doesn't require modification of
bootup scripts, so it works with any Linux distro.  It's a bit messy to
build and maintain, though.  'tmpfs' is cleaner, but it requires custom
script very early in the boot process.  On Slackware, it's no problem;
on other distro, however, it's not that easy.

-- 
William Park <opengeometry-FFYn/CNdgSA at public.gmane.org>, Toronto, Canada
Slackware Linux -- because I can type.
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