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Re: Integrate Knoppix in Debian (was: Re: Debian Enterprise?)

Matt Zimmerman <mdz@debian.org> writes:

> On Wed, Nov 26, 2003 at 01:46:28AM +0100, Goswin von Brederlow wrote:
> > Debix (through the device mapper) completly removes the live CD aspect of
> > the installation. The CD part becomes completly transparent. The Knoppix
> > developers are quite intrested in this since all the other methods
> > (onionfs, lucientfs, symlink farm) all, well, suck. When 2.6 is released
> > and becomes stable device mapper support is in the standard kernel so no
> > extra patches will be neccessary, which is another argument for it.
> So am I to understand that you plan to use device-mapper snapshots in order
> to provide copy-on-write at the block device level in order to implement
> this?  If so, this is interesting.  I assume you will use a RAM disk for
> writable storage?  What filesystem will you use?

Yes, thats exactly how it works. One device for the loopback file, one
for the ramdisk/tmpfs storage and a copy-on-write snapshot combining
those two into the real root device.

Recent kernels allow using loopback files on tmpfs. Older kernels need
to use one or more ramdisks.

The problem with ramdisks is that the default size is 4 MB and
specifying the ramdisk size as boot parameter is not allways
possible. Compiling it in is also bad. With tmpfs the size can be set
at runtime and if someone adds swap that will be used for the tmpfs as
well when needed.

As filesystem I only used ext2. But device mapper provides a normal
block device like any other. Only think to note is that using a
journaled filesystem like ext3, xfs or reiserfs might not be the best
idea since the journal will fill up 32 MB (or whatever the fs has) of
ram with no benefit.

On the other hand if harddisk install is to be suported using xfs
would have the big advanted that one can move the device snapshot onto
the harddisk and grow the filesystem to fit. The moving is done with a
kernel thread beneeth the block device and at some point the data will
have moved from the cdrom and ramdisk to harddisk and the
cdrom/ramdisk can be ejected/freed. You never have to reboot or
otherwise stop your work. At some point it will just tell you its


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