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Re: knoppix to debian again

Yves Combe <yves@ycombe.net> writes:

> Hello,
> 6) device-mapper. I don't understand what you do: you put all the
> livecd system in ram (like toram knoppix option?). that needs 2G ram?

No. The device mapper thing is something special to Debix.

Device-mapper (the kernel part of lvm2 and evms2) allows to make
writeable snapshots. Snapshots are copy-on-write so they only use as
much space as blocks are changed.

The trick Debix uses is to setup a writeable snapshot using a loopback
file on the CD and a ramdisk. That makes the loopback file appear to
be writeable like any normal block device. Given 1GB of ram you have
about 250000 4K writes before you run out. If you change the same
block multiple times that still counts only once.

The big benefit of this method is that the only thing handling the
live-cd aspect (i.e. that you have a read-only medium) is the initial
ramdisk. The rest of the system doesn't have to change a thing. In
fact I boot a normal Debian created by cdebootstrap with it.


PS: With a little extra C programm you can move the data from the CD
to another device on the fly (hd-install) or from the ramdsik to
another device (save changes). But thats not yet included.

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