Hi,
I talked a lot with some LVM people and a Knoppix developer about
using some new LVM features for a live CD and debian-installer at the
Linuxtag a few weeks back. Since then I created a test CD that
implements a few of those features as feasability study and it works.
So what am I talking about anyway?
1. lvm maps physical devices onto virtual devices. One can combine and
mix several physical devices into one logical one
2. lvm can do writeable snapshots. One can have a (even read-only)
block device and make a snapshot of it where one can write to like any
normal device but it will only store changes to the original device
(or whenever the original device gets changed it saves the original
data too)
3. lvm can duplicate and move chunks from one physical device to
another on the fly while the logical devices are still in use.
What the hell would debian-installer need that for?
1. One can create a seemingly writeable loopback filesystem on CD. The
CD holds a readonly loopback file and one can create a snapshot of
that together with a ramdisk or shmfs (or on newer kernels a file
on tmpfs). Any writes to the loopback file then go into the ramdisk
and are lost on reboot. I have a CD image with a 100% normal woody
system on it with a initrd that sets such a snapshot up. Works like
a charme.
With this instead of installing all (selected) udebs into a big
ramdisk the udebs can be preinstalled into a loopback file upon cd
creation and mounted this way to reduce the ram consumtion a
lot. No complicated linkfarms would be neccessary. It would also
instantly have all udebs unpacked already so its faster too. The
Installer would then just run the postinst files when an udeb is
selected.
2. The CD could contain a knoppix like live filesystem that
autodetects most stuff and asks questions about the rest and thus
configures itself. Then once the harddisk is partitioned and
formated the running live filesystem can be transfered over to
harddisk on the fly in the background and the user can just keep on
working all the time and doesn't need to reboot even once.
3. When repartitioning a harddisk one has to reboot if any partition
is in use to let changes take affect. Using lvm the partition can
be mapped seperatly and those mappings can be changed in
userspace. One can also move partitions around in the background
with lvm and even have a resume feature when the system crashes
inbetween (provided there is some space to store resume
infos). IIRC evms has some of those features already.
4. And another realy strange feature: Multi-install. One can stick say
8 harddisks into one system, boot the debian-installer and set
those harddrives up as mirrors of each other. Then one installs as
allways and when done distributes the harddisks to 8 computers to
get 8 identical systems. Ok, realy strange features but might be
usefull for pool installs. Maybe network block devices could be
used instead of removing the disks.
So what do you think (mainly about the first 2 points)?
A test CD image with the woody live cd is available on
rsync://mrvn.homeip.net/images/
but beware, its a DSL line so the 50MB bz2 will take ages.
MfG
Goswin
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