Synology multi-drive systems offer RAID capability, RAID is an ancronym that stands for Redundant Array of Independent Disks. What RAID offers to the user is performance, redundancy, or a combination of both.
Currently, our systems offer RAID Levels 0, 1, 5
For the sake of this discussion, we will be using 80GB drives.
RAID Level 0 offers no redundancy, but offers great throughput. It requires a minimum of two disks, and data will be "stripped" or written to both disks at the same time. This is great if you require pure performance over hardware failure. Theoretically speaking, if you have two SATA150 drives in a RAID 0 Array, the data will be written at 300MB/s. Total capacity offered, 160GB
RAID Level 1 offers redundancy over performance and it requires a minimum of two disks. This is great for server applications or backup servers. What happens is the same data is "mirrored" across both disks at the same time. Because of the nature of this array, should one disk fail, you still have a copy of your data on the redundant drive. Total Capacity offered, 80GB
RAID Level 5 offers a combination of both performance and redundancy and requires a minimum of 3 disks, where the data and parity information will be written across all 3 drives. However, the parity information (used for data recovery if the array fails) is not on one disk, it's across all 3, so, should 1 disk fail, the other two disks contain information necessary to rebuild the third disk. Capacity available, 160GB
Read more about RAID here.