The installer now wants to know which partitions to use for the mirror. The device names that it gives are a bit strange; it's using devfs, which is a now deprecated Linux subsystem that allows /dev device trees to be build automatically.
Select the two partitions that you want to use for this first mirror, and then select "Continue".
If you're creating more than one set of mirrored partitions, select "Create MD Device" again and repeat this process. Otherwise, select "Finish".
Now we're back at the disk overview, and we can see our new RAID1 device, in addition to the physical partitions. Now we want to put a filesystem onto our raid device and assign it to a point in the filesystem, so cursor down and select the contents of the RAID device:
Select "Use as".
Choose your filesystem type. I have selected Ext3. You may also consider using ReiserFS or XFS. Be aware, however, that if you're planning on using Grub as a boot manager, it might have problems recognising XFS partitions.
We want to make sure that a new filesystem is formatted onto the disk, so make sure that the "Format the partition" option is set to "yes, format it".
Finally, set the mount point for the filesystem. In my case, this is the root partition. Once all that is done, select "Done setting up the partition".
You will now see that your mirrors have a filesystem assigned to them, and that they have mount points. When ready, select "Finish partitioning and write changes to disk".
Now it will confirm that you want to do this. Select "Yes", and it will write the changes to your disks, destroying any data that might have been there beforehand:
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