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+Title: Setting up a software RAID1 
+Date: 2017-07-12
+Lang: en
+Tags: Gentoo
+
+I'm helping a friend to set up a machine with [Gentoo](http://www.gentoo.org). This will be a machine to host production applications and it is equipped with 2 identical hard drives. I will set up a [software RAID 1](https://raid.wiki.kernel.org). This has been a life saver on a similar machine before where one of the drives failed and we had no data loss and only minimal downtime during which the drive was replaced.
+
+One goal of the new setup is to remain bootable even if one of the drives fails. I had trouble accomplishing this in earlier setups so this time I tested the process locally on a virtual machine before setting up the real iron.
+
+The first step of the setup is partitioning the drives. The [handbook](https://wiki.gentoo.org/wiki/Handbook:AMD64/Installation/Disks) suggests adding a small partition at the beginning of the drive to enabl booting from a gpt partitioned drive. The `/boot` partition will be formatted using [ext4](https://en.wikipedia.org/wiki/Ext4) because the filesystem will remain bootable even if one of the drives is missing. The rest of the disk will be formatted using [xfs](https://en.wikipedia.org/wiki/XFS). To recap the layout:
+
+    Number  Start    End      Size     File system  Name    Flags
+     1      1.00MiB  3.00MiB  2.00MiB               grub    bios_grub
+     2      3.00MiB  95.0MiB  92.0MiB               boot
+     3      95.0MiB  8191MiB  8096MiB               rootfs  raid
+
+The second drive is partitioned exactly the same.
+
+Now let's create a RAID 1 for the boot partition:
+
+    mdadm --create /dev/md0 --level=1 --raid-devices=2 /dev/sdb2 /dev/sdc2
+
+and for the rootfs:
+
+    mdadm --create /dev/md1 --level=1 --raid-devices=2 /dev/sdb3 /dev/sdc3
+
+To maintain the RAID device numbering even after reboot, the RAID config has to be saved. This will be done by
+
+    mdadm --detail --scan >> /etc/mdadm.conf
+
+Then create an ext4 filesystem on `/dev/md0` and an xfs filesystem on `/dev/md1`. Nothing noteworthy here.
+
+The observant reader will have noticed from the device names above that I'm testing my installation from a running system on `/dev/sda`. To save the hassle of going through the entire stage3 setup process I'm simply copying the running system to the newly created RAID filesystems.
+
+After chrooting into the new system some changes have to be made to the [genkernel](https://wiki.gentoo.org/wiki/Genkernel) config in order to produce a RAID enabled initramfs. In `/etc/genkernel.conf` set
+
+    MDADM="yes"
+    MDADM_CONFIG="/etc/mdadm.conf"
+
+Now we're set to build the kernel.
+
+While it's compiling, edit `/etc/default/grub` (I'm of course using [grub2](https://www.gnu.org/software/grub/manual/grub.html) for booting)  and add
+
+    GRUB_CMDLINE_LINUX="domdadm"
+
+Setup grub on both devices individually using `grub-install /dev/sdb` and `grub-install /dev/sdc`.
+
+After the kernel has finished compiling, generate the proper grub config using `grub-mkconfig -o /boot/grub/grub.cfg`.
+
+Before rebooting the `fstab` has to be set up correctly. I prefer to use UUIDs for filesystems instead of device names - this is a bit more fault tolerant in case device numbering is mixed up. It was a main challenge to find an unambiguous UUID for the both RAID filesystems. There are a number of places to get a UUID from: `mdadm --detail`, `blkid`, `tune2fs -l`, `xfs_admin -u` and I'm sure I forgot more. The helpful guys at the [gentoo IRC channel](https://www.gentoo.org/get-involved/irc-channels/) pointed me in the right direction. Use `lsblk -f /dev/md0` to find a UUID that uniquely identifies the RAID and check using `findfs UUID=<uuid>`.
+
+After updating the `fstab` the system is ready for a first boot into the RAID setup.
+
+I tested failing drives by simply removing the first (or the second) drive from the virtual machine. The whole setup still boots off either drive.