Software RAID on Gentoo and Debian

Up until now I backed up my hard drive with important files with unison. I did that by regularly plugging in a similar second hard drive into one of my empty drive bays, running unison, waiting for it to finish and finally pulling the new hard drive out again to store it on my shelf. This obviously spares at least one drive from running all the time but is a very tedious task, especially if it's IDE drives which aren't hot-swappable.

So I've recently gone through the very unpleasant course of loosing the data on one of my old IDE hard drives, of course the one which I don't back up with unison but located in the same file server. Rescuing data wasn't possible, even forensic tools like foremost and scalpel couldn't retrieve all the files, e.g. mp3 files were all either 7.2MB or 42MB in size and contained everything but a valid music stream. foremost didn't even finish and segfaulted after carving about 30% of the disk.

To make life easier and to possibly prevent such tragedies from happening in the future, I ordered two identical Seagate SATA hard drives, each 500GB. While Seagate doesn't precisely build silent drives but grants five years of warranty, these seemed to be perfect for the file server which is running in the cellar. But while planning on using RAID1 for the Seagate drives, I decided to do the same with my workstation - but this time not for reasons of redudancy but for the sake of access speed - and bought two Western Digital HDDs of the same size which promised to be less loud and also had 16MB of cache (instead of 8MB on the Seagate disks).

Once they arrived I added a spare PCI SATA RAID controller with a SiliconImage chipset into the file server and defined a RAID1 array in the card's BIOS setup tool. The stock Debian kernel contained all needed modules and after installing the dmraid with apt, I could view the setup

# dmraid -r
/dev/sda: sil, "sil_ahbgafcdfhah", mirror, ok, 976771120 sectors, data@ 0
/dev/sdb: sil, "sil_ahbgafcdfhah", mirror, ok, 976771120 sectors, data@ 0

# dmraid -s sil_ahbgafcdfhah
*** Active Set
name : sil_ahbgafcdfhah
size : 976771120
stride : 0
type : mirror
status : ok
subsets: 0
devs : 2
spares : 0

Mounting worked the usual way but instead of using the raw device node, you'd need to mount the respective mapped device, in my case /dev/mapper/sil_ahbgafcdfhah .

My workstation also has a (fake/software) RAID controller (onboard ICH7) but it wasn't as easy to set up because the RAID1 array acts as the boot drive and the kernel needs to be booted accordingly. Also, grub doesn't recognize RAID setups and thus I needed to create a separate small primary partition at the beginning of the RAID drive which is accessible, no matter if access it as a RAID drive or as its underlying disks. On this partition I store the kernel and its ramdisk, as well as grub's stage files. I did that by booting the Gentoo install CD, setting up dmraid as I did on the Debian box, formatting the RAID drive and copying the old drive's partitions with

# find . | cpio -pdum /mnt/target

The second problem was building a working kernel and its ramdisk. Custom built kernels didn't work because they're unable to initialize the mapping with dmraid. Luckily Gentoo's genkernel package saved my day.

# genkernel --dmraid all --menuconfig

After compiling the kernel in my chrooted target system, I copied the resulting files to my boot partition mentioned above and add the following entry to grub's config file:

title=Gentoo Linux 2.6.23 genkernel
root (hd0,3)
kernel /kernel-genkernel-x86_64-2.6.23-gentoo root=/dev/ram0 init=/linuxrc \
real_root=/dev/mapper/isw_bcaheaacjd_NewSystem2 dodmraid vga=792 ramdisk=8192
initrd (hd0,3)/initramfs-genkernel-x86_64-2.6.23-gentoo

Note that the fact that the root partition is the partition with number four has nothing to do with it being situated at the very beginning of the disk.