6.2.1. How Do I Do That?
RAID can be managed by the kernel, by the kernel working with the motherboard BIOS, or by a separate computer on an add-in card. RAID managed by the BIOS is called
Using
Add-in cards that contain their own CPU and battery-backed RAM can reduce the load of RAID processing on the main CPU. However, on a modern system, RAID processing takes at most 3 percent of the CPU time, so the expense of a separate, dedicated RAID processor is wasted on all but the highest-end servers. So-called RAID cards without a CPU simply provide additional disk controllers, which are useful because each disk in a RAID array should ideally have its own disk-controller channel.
There are six 'levels' of RAID that are supported by the kernel in Fedora Core, as outlined in Table 6- 3.
Table 6-3. RAID levels supported by Fedora Core
| RAID Level | Description | Protection against drive failure | Write performance | Read performance | Number of drives | Capacity |
|---|---|---|---|---|---|---|
| Linear | Linear/Append. Devices are concatenated together to make one large storage area (deprecated; use LVM instead). | No. | Normal. | Normal | 2 | Sum of all drives |
| 0 | Striped. The first block of data is written to the first block on the first drive, the second block of data is written to the first block on the second drive, and so forth. | No. | Normal to normal multiplied by the number of drives, depending on application. | Multiplied by the number of drives | 2 or more | Sum of all drives |
| 1 | Mirroring. All data is written to two (or more) drives. | Yes. As long as one drive is working, your data is safe. | Normal. | Multiplied by the number of drives | 2 or more | Equal to one drive |
| 4 | Dedicated parity. Data is striped across all drives except that the last drive gets parity data for each block in that 'stripe.' | Yes. One drive can fail (but any more than that will cause data loss). | Reduced: two reads and one write for each write operation. The parity drive is a bottleneck. | Multiplied by the number of drives minus one | 3 or more | Sum of all drives except one |
| 5 | Distributed parity. Like level 4, except that the drive used for parity is rotated from stripe to stripe, eliminating the bottleneck on the parity drive. | Yes. One drive can fail. | Like level 4, except with no parity bottleneck. | Multiplied by the number of drives minus one | 3 or more | Sum of all drives except one |
| 6 | Distributed error-correcting code. Like level 5, but with redundant information on two drives. | Yes. Two drives can fail. | Same as level 5. | Multiplied by the number of drives minus two | 4 or more | Sum of all drives except two |
For many desktop configurations, RAID level 1 (RAID 1) is appropriate because it can be set up with only two drives. For servers, RAID 5 or 6 is commonly used.
Although Table 6-3 specifies the number of drives required by each RAID level, the Linux RAID system is
