Saturday, November 13, 2010

Debian from scratch on lvm2 and software raid

Debian usually comes with a great installer, that enables you to use menu-based configuration tools to setup many usefull features. Among them are also lvm2 and software-raid, using mdadm. But if you want to install Debian from scratch, using debootstrap, you have to setup these features youself and if you want a root partition on lvm and raid, you need to consider a few things, so your system will be able to boot.

Debian does not use any custom “boot flags”, as Gentoo does, where you specify “dolvm, domdadm” as kernel parameters in grub configuration, but offers a tool to create a ramdisk, suited for the job. $update-initramfs can be called via command line, but first, some settings need to be made. update-initramfs will read /etc/mdadm/mdadm.conf to retrieve configuration so the raid arrays can be assembled. Basically you want to have something like this

———snip———-
ARRAY /dev/md0 metadata=1.0 devices=/dev/sda1,/dev/sdb1
ARRAY /dev/md1 metadata=1.1 devices=/dev/sda2,/dev/sdb2
———snap———-
in your mdadm.conf.

Further more, update-initramfs will look into /etc/fstab and /boot/grub/menu.lst to gather information about the root device/partition. I fiddled a bit here, but in the end, it seemed, that devices, whose paths contain “mapper” are indentified as logical volumes, thus enabling lvm on boot. I tried
——————–
menu.lst
kernel /vmlinuz-xxx root=/dev/system/root ro quiet
——————–
fstab
/dev/system/root / ext3 defaults 0 1
——————–
first, but that didn’t work. So i put it this way
——————–
menu.lst
kernel /vmlinuz-xxx root=/dev/mapper/system-root ro quiet
——————–
fstab
/dev/mapper/system-root / ext3 defaults 0 1
——————–
and my system would boot. With “system” being my volume group, you basically need a path of this scheme: /dev/mapper/[volume-group]-[volume] in both /etc/fstab and /boot/grub/menu.lst.

That done, run either $update-initramfs -u if you want to update an existing ramfs or create a new one, using $update-initramfs -c -k . The version-label can is only a name and can entirely be made up.

Author: Fabian Sch├╝tz

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