Managing RAID on Linux

RAID-4

RAID-4 stripes block-sized chunks of data across each drive in the array marked as a data drive. In addition, one drive is designated as a dedicated parity drive (see Figure 2-8).

Figure 2-8. RAID-4 stripes data to all disks except a dedicated parity drive.

RAID-4 uses an exclusive OR (XOR) operation to generate checksum information that can be used for disaster recovery. Checksum information is generated during each write operation at the block level. The XOR operation uses the dedicated parity drive to store a block containing checksum information derived from the blocks on the other disks.

XOR

The exclusive OR (XOR) is a logical operation that returns a TRUE value if and only if one of the operands is TRUE. If both operands are TRUE, then a value of FALSE is returned.

p     q     p XOR q
-----------------------
T     T     F
T     F     T
F     T     T
F     F     T

When a parity RAID generates its checksum information, it performs the XOR on each data byte. For example, a RAID-5 with three member disks writes the byte 11011011 binary to the first disk and the byte 01101100 to the second disk. The first two bytes are user data. Next, a parity byte of 10110111 is written to the third disk. If a byte is lost because of the failure of either the first or the second disk, the array can perform the XOR operation on the other data byte and the parity information in order to retrieve the missing data byte. This holds true ...