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RAID-1 provides complete fault tolerance, it is cost-prohibitive for some users because it at least doubles storage costs. However, for sites that require zero downtime, but are willing to take a slight hit on write performance, mirroring is ideal. Such sites might include online magazines and newspapers, which serve a large number of customers but have relatively static content. Online advertising aggregators that facilitate the distribution of banner ads to customers would also benefit from disk mirroring. If your content is nearly static, you won't suffer much from the write performance penalty, while you will benefit from the parallel read-as-you-serve image files. Full fault tolerance ensures that the revenue stream is never interrupted and that users can always access data.

RAID-1 works extremely well when servers are already load-balanced at the network level. This means usage can be distributed across multiple machines, each of which supports full redundancy. Typically, RAID-1 is deployed using two-disk mirrors. Although you could create mirrors with more disks, allowing the system to survive a multiple disk failure, there are other arrangements that allow comparable redundancy and read performance and much better write performance. See the Hybrid Arrays section, later in this chapter. RAID-1 is also well suited for system disks.

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.

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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 for any number of data bytes or, in our case, disks.

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In the event of a disk failure, an XOR operation can be performed on the checksum information and the parallel data blocks on the remaining member disks. Users and applications can continue to access data in the array, but performance is degraded because the XOR operation must be called during each read to reconstruct the missing data. When the failed disk is replaced, administrators can rebuild the data from the failed drive using the parity information on the remaining disks. By sequentially performing an XOR on all parallel blocks and writing the result to the new drive, data is restored.

Although the original RAID specification called for only a single dedicated parity drive in RAID-4, some modern implementations allow the use of multiple dedicated parity drives. Since each write generates parity information, a bottleneck is inherent in RAID-4.

Placing the parity drive at the beginning of an I/O channel and giving it the lowest SCSI ID in that chain will help improve performance. Using a dedicated channel for the parity drive is also recommended.

It is very unlikely that RAID-4 makes sense for any modern setup. With the exception of some specialized, turnkey RAID hardware, RAID-4 is not often used. RAID-5