Re: [Morphix-devel] UnionFS branches

[Alex de L. <al...@de...>]

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Hi Nextime, (sorry for the delay until posting, mailman complained about
your message's subjectline)

The reason for the '--after 0' parameter was because minimod unionfs
branches are read-only. By forcing precedence of the mainmod branch the
whole filesystem remains read/writable.

Note that I haven't thoroughly tested this, so I might be wrong.
Naturally a minimodule would ideally 'overwrite' mainmodule files, not
doing so is needlessly confusing, but afaik it isn't possible without
also blanking out the rw-layer.

Having an option to set the minimod layer position seems reasonable.
Doing so would mean an un-writable filesystem if the above is correct,
but it would at least allow flexibility in this department.

Re-reading your post, I think that if we changed '--after 0' to '--after
1', the result would allow minimod precedence over the mainmod, and
still the filesystem would be writable. Do you think this is correct?

cheers & thanks for looking into this,

Alex

ne...@ne... wrote:
> First of all, sorry for the wrong post on the "cvs" mailing list!
> 
> The message was:
> 
> Ciao all.
> 
> I think that MiniModule unionfs overlaying is conceptually wrong.
> 
> in the base module we have 2 init script that mount main and mini
> modules by using unionfs.
> 
> While Mainmodule is overlayed in morphix-start in this way:
> 
>      mount -t unionfs -o dirs=/mnt/main${UNIONFS_MOUNT_OPT},debug=$DEBUGOVERLAY none /mnt/main
>      unionctl /mnt/main --add --mode ro /mnt/main_ro
>      unionctl /mnt/main --add --mode rw $DIR2/root
> 
> MiniModules are overlayed in load-minimodules.sh with:
> 
>    if [ -d /mnt/main/mnt/mini/mod$count/root ]; then
>        unionctl /mnt/main --add --after 0 --mode ro /mnt/main/mnt/mini/mod$count/root
>    fi
> 
> This make the root file system on the resulting live cd booted like
> this:
> 
> root@Medianix:~# unionctl / --list
>         /mnt/.hidden_base/ramdisk/var/tmp/overlay/root (rw-)
>         /mnt/mini/mod2/root (r--)
>         /mnt/.hidden_base/mnt/main_ro (r--)
> root@Medianix:~#
> 
> 
> as the "0" branch is the rw mounted ramdisk, so, when add to the unionfs
> the MiniModule, it will go after this and before the Main that is
> /mnt/.hidden_base/mnt/main_ro.
> 
> In this way if we have 2 files (let me say for an example, /etc/passwd)
> both in main *and* minimodule, the main one take precedence over the
> minimodule one.
> 
> While having multiple minimodule with the same file can open some
> issues, i think that it will be better to permit to minimodules
> to take precedence over the mainmodule, so, i think that in
> load-minimodules.sh the unionctl line shuld be
>    unionctl /mnt/main --add --mode ro /mnt/main/mnt/mini/mod$count/root
> without the --after 0 switch.
> 
> Another idea is to use a file inside the minimodule, something like
> "unionfsposition" that contain the position to insert the minimodule
> branch, letting us configure this from the minimodule itself. Something
> like:
> 
> if [ -d /mnt/main/mnt/mini/mod$count/root ]; then
>   if [ -f /mnt/main/mnt/mini/mod$count/unionfspos ]; then
>     ufsafter="--after `cat /mnt/main/mnt/mini/mod$count/unionfspos`"
>   fi
>   unionctl /mnt/main --add $ufsafter --mode ro /mnt/main/mnt/mini/mod$count/root
> fi
> 
> 
> 
> 
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