English, français
y español
Loads of very detailed info are available on Wikipedia. Just in case, it means: Redundant Array of Independent Disks.
More or less, you can create arrays of hard drives to make things run faster, safer, bigger (pick what you need!) The safer is what I use, it is RAID1. The faster is RAID0 and if any one drive dies, bye bye your data! And the bigger is RAID5. That's what Linux offers, so you do not have to waste your time reading about the others.
Not too surprising, it's not exactly straight forward. It requires three steps and these are not exactly in order.
First of all, select the "Manually edit the partition table" when asked how you want to partition disks.
Now, you see the drives. If you do not need the data left on the drive (easier!) then just delete any existing partitions.
Now define a list of partitions that you want to create. This is important because you will do it twice. There is an example of default partitions:
/ /boot /home swap /tmp /usr /var
I usually only use /, /home, /tmp, /var and of course a swap partition. Just in case.
The /tmp and swap, I do not create as RAID to safe time. Also, the /tmp partition, these days, often remains in memory only. So if you have a failure, you are good for a reboot in many cases.
Once you know the partitions, define the size (amount of Mb, Gb, Tb, Pb you want to assign to each partition.) Note that you do that only once since you will be duplicating that data on both systems.
Okay, now you can start creating the partitions. You can always create them in the order you defined them, but I suggest putting the /tmp and swap partitions in the middle of the drive. That way you have less head work (if you do not yet have Flash drives that is.)
Now, you said "Create new partition". I suggest you use 3 Primary partitions, 1st one / (or /boot) and mark it as boot (i.e. it will boot). Then change the file system type from Ext3 journaling file system to Physical volume for RAID. This may feel really weird since the partition becomes what I would call "not much". In other words, it does not give you a way to assign a directory and other format and mount point information.
Don't worry, this is the next step. Repeat this step for each partition and don't forget that having the swap on RAID is not very likely to work. So if you want to get more swap or just make it faster, avoid the duplication (it is faster to write, to read it can be slower but that will depend on your hardware.) Anyway, if you swap a lot, you may want to extend your RAM.
Now you are done with the partitions, it should all say "K raid". Now look closely in the menu, at the very top (yep! usually well hidden), you see the menu: Configure Software RAID.
This option will let you attach the different partitions together. So if you created the exact same disks on, say, sda and sdb, you will attach sda1 with sdb1, sda2 with sdb2, etc.
Note that only RAID partitions are shown. Also, partitions already attached together do not show up as a choice for the following attachments.
Once all the RAID paritions are attached, exit that menu and you will now see a new set of entries in the list of drives. These are the software RAID drives. Hit Enter on them to edit their info.
Yes! This step looks like a normal partition setup. Except that by default it says "do not use" instead of "Ext3 ...". Change the partition type and you get the mount point, the different modes, etc. Change to your liking.
Finish the installation and enjoy!
Ah! If you want to install GRUB on the 2nd drive, since that's not done automatically, you may want to do some extra work. For more info, look at Derrick Webber article on how to 100% finish up the installation.
Also, you may want to test, test, test. Just in case.