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Disk Shadowing or Mirroring 

If you will have mission-critical applications that you absolutely cannot allow to go down, consider disk shadowing or mirroring. As the term implies, disk shadowing or mirroring is the process whereby each disk has a shadow or mirror disk to which data is written simultaneously. This redundant storage allows the shadow disk or set of disks to pick up the load in case of a disk crash on the primary disk or disks; thus the users never see a crashed disk. Once the disk is brought back online, the shadow or mirror process brings it back in sync by a process appropriately called resilvering. This also allows for backup since the shadow or mirror set can be broken (e.g., the shadow separated from the primary), a backup taken, and then the set resynchronized. I have heard of two, three, and even higher-number mirror sets. Generally, I see no reason for more than a three-way mirror as this allows for the set of three to be broken into a single and a double set for backup purposes. Shadowing or Mirroring is RAID1.

The main disadvantage to disk shadowing is the cost: For a 200-GB disk “farm,” you need to purchase 400 or more gigabytes of disk storage.

Redundant Arrays of Inexpensive Disks (RAID)

The main strength of RAID technology is its dependability. In a RAID5 array, the data is stored, as is parity data and other information about the contents of each disk in the array. If one disk is lost, the others can use this stored information to re-create the lost data. This makes RAID 5 very attractive. RAID 5 has the same advantages as shadowing and striping but at a lower cost. It has been suggested that if the manufacturers would use slightly more expensive disks (RASMED—redundant array of slightly more expensive disks) performance gains could be realized. A RAID 5 system appears as one very large, reliable disk to the CPU. There are several levels of RAID to date:

   RAID0. Known as disk striping.

   RAID1. Known as disk shadowing.

   RAID0/1. Combination of RAID0 and RAID1. May also be called RAID10 depending on whether they are striped and mirrored or mirrored then striped. It is generally felt that RAID10 performs better than RAID01.

   RAID2. Data is distributed in extremely small increments across all disks and adds one or more disks that contain a Hamming code for redundancy. RAID2 is not considered commercially viable due to the added disk requirements (10 to 20 percent must be added to allow for the Hamming disks).

   RAID3. This also distributes data in small increments but adds only one parity disk. This results in good performance for large transfers; however, small transfers show poor performance.

   RAID4. In order to overcome the small transfer performance penalties in RAID3, RAID4 uses large data chunks distributed over several disks and a single parity disk. This results in a bottleneck at the parity disk. Due to this performance problem, RAID4 is not considered commercially viable. RAID3 and -4 are usually are used for video streaming technology or large LOB storage.

   RAID5. This solves the bottleneck by distributing the parity data across the disk array. The major problem is that it requires several write operations to update parity data. That said, the performance hit is only moderate, and the other benefits may outweigh this minor problem. However, the penalty for writes can be over 20 percent and must be weighed against the benefits.

   RAID6. This adds a second redundancy disk that contains error-correction codes. Read performance is good due to load balancing, but write performance suffers because RAID6 requires more writes than RAID5 for data update.

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