Take a crash course in Ubuntu server administration and learn specific details that set Ubuntu Server apart from other platforms in this chapter from The Official Ubuntu Book.
Which RAID to Choose? If you're indecisive by nature, the past few paragraphs may have left you awkwardly hunched in your chair, mercilessly chewing a No. 2 pencil, feet tapping the floor nervously. Luckily, the initial choice of RAID level is often a no-brainer, so you'll have to direct your indecision elsewhere. If you have one hard drive, no RAID for you. Do not pass Go, do not collect $200. Two drives? Toss them into RAID 1, and sleep better at night. Three or more? RAID 5. Unless you really know what you're doing, avoid RAID 0 like the plague. If you're not serving mostly read-only data without a care about redundancy, RAID 0 isn't what you want.
Other RAID Modes
Though the installer offers only the most common RAID modes—0, 1, and 5—many other RAID modes exist and can be configured after the installation. Take a look at http://en.wikipedia.org/wiki/RAID for a detailed explanation of all the modes.
Setting Up RAID
After carefully studying the last section, maybe reading a few books on abstract algebra and another few on finite field theory, you finally decided on a RAID level that suits you. Since books can't yet read your mind, we'll assume you chose RAID 1. So how do you set it up?
Back to the installer. When prompted about partitioning disks, you'll want to bravely select the last option, Manually Edit Partition Table.
Below the top two options on the screen (Guided Partitioning, and Help), you'll find a list of the physical drives in your server that the Ubuntu installer detected.
Avoiding the "Oh, No!" Moment
We've said this before, and we'll say it again: It's very easy to mistakenly erase valuable data when partitioning your system. Since you're installing a server, however, we'll assume you're comfortable deleting any data that might already exist on the drives. If this is not the case, back up all data you care about now! We mean it.
Indented below each drive, you'll find the list of any preexisting partitions, along with their on-disk ordinal number, size, bootable status, filesystem type, and, possibly, their mount point. Using the arrow keys, highlight the line summarizing a physical drive (not any of its partitions), and hit Enter—you'll be asked to confirm replacing any existing partition table with a new one. Select Yes, and the only entry listed below that drive will be FREE SPACE. In our fictional server, we have two 80GB drives—hda and hdb—so we'd follow this process for both drives, giving each a fresh partition table. Say we've decided on a 20GB /home partition. Arrow over to FREE SPACE, hit Enter, and create the partition. Once you've entered the size for the new partition, you'll be brought to a dialog where you can choose the filesystem and mount options. Instead of plopping a filesystem on the raw partition, however, you'll want to enter the Use As dialog and set the new partition to be a physical volume for RAID.
Still with us? Now rinse and repeat for the other drive—create the exact same partition, same size, and set it as a RAID volume. When you're done, you should be back at the initial partitioning screen, and you should have an identically sized partition under each drive. At this point, choose Configure Software RAID at the top of the screen, agree to write out changes to the storage devices if need be, and then choose to create an MD (multidisk) device. After selecting RAID 1, you'll be asked to enter the number of active devices for the array. In our fictional two-drive server, it's two. The next question concerns the number of spare devices in the array, which you can leave at zero. Now simply use the spacebar to put a check next to both partitions that you've created (hda1 and hdb1), and hit Finish in the Multidisk dialog to return to the basic partitioner.