Partitioning with fdisk
You don’t need to partition disks before using them in an array, but partitioning does provide a couple of advantages. First, partitioning is necessary if you want the kernel to automatically start arrays, because the md driver uses the partition type to identify member disks. Second, md devices don’t support partitioning directly, but in some cases, having a filesystem that spans an entire array is undesirable. Using software RAID for system partitions means that smaller partitions are necessary. After all, you don’t want /var or /boot to span a whole array.
If you have a lot of disks, then you might not want to go through the trouble of partitioning each disk—a process that can take a lot of time if you have more than a few drives. In that case, you can simply use a whole, unpartitioned disk as an array member (/dev/sda, for example). This means that you won’t be able to autostart arrays, however, so you’ll have to include commands to start md devices in your system initialization scripts.
The rest of this chapter assumes that disk-sized partitions are used, but unless you want to subpartition or need to automatically start arrays, you can skip the rest of this section. Replace the use of partitioned disks found in the examples throughout the rest of this chapter with unpartitioned devices. Where I use /dev/sdb1 to denote a single disk-sized partition as an array member disk, you could simply use /dev/sdb.