Unionfs

Forum: DSL Ideas and Suggestions
Topic: Unionfs
started by: friedgold

Posted by friedgold on May 05 2005,14:34

Just wondering if anyone has considered adding unionfs to DSL for future versions? Unionfs is (from its website) 'a stackable unification file system, which can appear to merge the contents of several directories'. At the moment DSL copies entire directories to the ramdisk (/home, /opt and if you load a .dsl /usr, /bin, etc) and replaces the original directories with sym link pointing to the ramdisk. With unionfs we could just overlay a ramdisk over the original filesystem. You then only need to store the changes you've made in the ramdisk, which is automatically overlayed the original read only filesystem.

Since only the changes are stored in the ramdisk, much more of the filesystem could be made writable without the current memory hit of running /etc/init.d/mkwritable. A number of other Linux livecd use unionfs successfully including Knoppix 3.8.

There's also ovlfs (Overlay filesytem) which provides a similar system

Posted by friedgold on May 06 2005,09:27

Does no one have any opinions on this??? Positive or negative?

Here's what I see as the advantages

-Should be able to use existing extensions without them having to be remade. .tar.gz and .dsl extensions can be extracted into a ramdisk overlayed over the root filesystem. .uci images can be mounted and overlayed straight over the root filesystem.

-Increased flexibililty by making the whole filesystem writable. For example you would no longer be restricted to just /opt and /home when backing up files in your filetool.lst. Also .uci type extensions could have the restriction of only having files in /opt /home lifted.

-Reduced memory usage. By only keeping the changes to the root filesystem in the ramdisk memory usage should be reduced. Also I expect there would be more .uci type extensions created, as without the restriction to /opt /home pretty much any .dsl could be easily converted to a .uci extension. This in turn should again help out memory usage.

-Unionfs is used in Knoppix 3.8, which can be used as a guide for implimentation

Disadvantages

-Obviously not a minor change, would require a lot of work
- ?

Posted by roberts on May 06 2005,11:56

Since you are looking for a response...
Our system was designed, built, and impemented before this was available.
Some of the best parts of our system (mountable apps) are now being implemented in other systems. I know of unionfs. It will be a fair amount of work plus the space requirements of is modules. So many demands only so mnay hours in a day. But it, as well as many other things, are on to be considered list.

Posted by friedgold on May 06 2005,21:42

Thanks for your reply. I certainly appreciate that adding unionfs support would be quite a undertaking. I just thought I'd suggest it as I couldn't find any simliar post via the search, and it seems like such a elegent solution to making the filesystem of a bootable cd writable.

Damn Small Linux is definately the best Linux Live CD I've come across, with or without unionfs support and I'd like to thank you (and the other developers) for putting together such a great distribution.

Posted by roberts on May 06 2005,22:02

Thanks for your comments and suggestion. It is really a matter of priorities.
And that is something that we will have to kick around to see what will be delivered next.

Posted by mikshaw on May 06 2005,23:50

just a FYI...when you run mkwriteable the contents of the system are not copied into ramdisk....it's mostly just symlinks. For the most part the actual files are still located in /KNOPPIX. The difference is that the individual files are now linked, rather than just the base directories, so adding to the dirs is possible.
Although there is an increase in RAM usage, it's not as bad as you may think.