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(?) The Answer Guy (!)

By James T. Dennis, tag@lists.linuxgazette.net
Starshine Technical Services, http://www.starshine.org/

(?) Translucent, Overlay, Loop, and Union Filesystems

From c17h21no4 on Wed, 02 Dec 1998

(?) Where can i find information/documentation about the loopback filesystem and the translucent file sytstem under linux. From what i see on the mail lists there is support but the links are old or outdated (Ben's link) and i seem to not be finding any info on it.

(!) According to an old version of the CD-ROM Howto:
Once upon a time there was an IFS (inheriting filesystem). This was written by Werner Almesberger for Linux version 0.99p11 was similar in principle to the "translucent fs" from Sun. This was a "copy-on-write" system, sometimes referred to as an "overlay" or "union" fs.
All of these are different terms for the same concept, you mount two (or possibly more) filesystems on the same point. Accessing files under these mount points is presents files from one of the underlying filesystems.
The most common case would be to lay a CD-ROM fs over a normal (ext2, minix, xiafs) filesystem. Any files on the "normal" (read-write) fs take precedence over any file with a colliding name on the CD-ROM. Any write attempt of a file results in a copy (or possibly a "diff" on a log-structured fs). Later access to such files will refer to the copy rather than the original.
An early version of the Yggdrasil Plug-n-Play Linux (*) distribution supported this (IFS) as an installation method, if I recall correctly.
As far as I know Werner's IFS hasn't been updated in years and there isn't any support for any of these union/translucent etc fs variants in the standard kernel. I did find on pretty obscure set of patches that appear to provide "overlay" filesystem support for 2.0.31 kernels at:
LOFS Patches for Linux:
... this has no README files or other documentation so my guess about their intent is purely from reading the patches. I think "Blah" in this URL refers to Mr. Benjamin LaHaise who apparently wrote the following to the Linux-Kernel mailing list in May of 1997:
> Now is a very good time to tell me if
> someone else has already got a working lofs :-)
I wrote one quite some time ago, and finally made patches against 2.0.30 last week. They're at ftp://dot.superaje.com/pub/linux/lofs-2.0.30.diff It's not perfect, but it works. (I do have a fancier 2.1.x version, but it'll be a while before i get anymore work done on it.)
This was in response to a Mr. Jon Peatfield's query. (The ftp link therein does not work). He mentioned some additional work on his 'lofs' as late as August of '97 --- quoted in a response by Linus regarding some VFS semantics.
I presume this is the "Ben" to which you are referring. I've blind copied his last known @ddresses. (Sorry if you get three copies of this).
There's a similar concept called a "cachefs" and there's a couple somewhat different concepts called "loop" filesystems.
A Linux "loop" or "loopback" filesystem allows one to mount a regular file as a filesystem. This only works if the file is an image of a supported filesystem. Thus, if you have a boot diskette image you can mount it on /dev/loop0, 'cd' into the mount point and view the contents.
I've leard of another interpretation of the phrase "loop back filesystem" that involves remounting the same filesystem with different option at different mount points. Thus you might mount one at /usr with "read-only" options and somewhere else with read-write and no-exec" However, I don't know which versions of Unix use this and it doesn't seem to match the Linux implemtation at all.
It is possible to enable encryption on your loop devices using the 'losetup' command (see the man page in section 8). However, this is more of a proof of concept than a real utility. See my column last month for pointers to some real cryptography packages, or look at the "privacy protected disk driver" (ppdd) which is one I forgot to mention last month.
'cachefs' and 'tmpfs' are filesystems that are supported by Solaris.
The CODA project at http://coda.cs.cmu.edu also has some interesting replication and caching features.
Obviously when we start talking about specialized filesystems we see myriad terminology collisions and ambiguities.
For now I'd say that Linux LOFS/Translucent filesystems are not "ready for prime time." However, if you're interested in working on the code --- go for it!

Copyright © 1999, James T. Dennis
Published in The Linux Gazette Issue 36 January 1999

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