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RAID - Redundant Arrays of Inexpensive Disks

Oracle Tips by Mike Ault

The main strengths of RAID technology are its dependability and IO bandwidth. For example, in a RAID5 array, the data is stored as are checksums 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. However, this rebuild of data on-the-fly causes a massive hit on performance. In RAID 1, RAID 10 and RAID 01 failed disks are immediately replaced by their mirror with no performance hit. This makes RAID very attractive. RAID 5 has the same advantages as shadowing and striping 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 system appears as one very large, reliable disk to the CPU. There are several levels of RAID to date:

·         RAID 0. Known as disk striping.

·         RAID 1. Known as disk shadowing or mirroring.

·         RAID 0/1. Combination of RAID0 and RAID1. May also be called RAID 10 depending on whether they are striped and mirrored or mirrored then striped. It is generally felt that RAID 10 performs better than RAID 01.

·         RAID 2. Data is distributed in extremely small increments across all disks and adds one or more disks that contain a Hamming code for redundancy. RAID 2 is not considered commercially viable due to the added disk requirements (10 to 20 percent must be added to allow for the Hamming disks).

·         RAID 3. This also distributes data in small increments but adds only one parity disk. This results in good performance for large transfers, but small transfers show poor performance.

·         RAID 4. 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 RAID 4 is not considered commercially viable. RAID 3 and 4 are usually are used for video streaming technology or large LOB storage.

·         RAID 5. This solves the bottleneck by distributing the parity data across the disk array. The major problem is it requires several write operations to update parity data. The performance hit is only moderate, and the other benefits may outweigh this minor problem. However the penalty for writes can be over 20% and must be weighed against the benefits.

·         RAID 6. This adds a second redundancy disk that contains error-correction codes. Read performance is good due to load balancing, but write performance suffers due to RAID 6 requiring more writes than RAID 5 for data update.

For the money, I would suggest RAID0/1 or RAID1/0, that is, striped and mirrored. It provides nearly all of the dependability of RAID5 and gives much better write performance. You will usually take at least a 20 percent write performance hit using RAID5. For read-only applications RAID5 is a good choice, but in high-transaction/high-performance environments the write penalties may be too high. Figure 18 shows RAID 1-0 or 0-1 depending on whether you stripe and then mirror or mirror first and then stripe. In most situations you get better performance from RAID 1-0 (mirroring then striping.)

Figure 18: Mirroring and Striping

Table 1 shows how Oracle suggests RAID should be used with Oracle database files.

RAID

Type of Raid

Control File

Database File

Redo Log File

Archive Log File

0

Striping

Avoid

OK

Avoid

Avoid

1

Shadowing

Best

OK

Best

Best

0+1

Striping and Shadowing

OK

Best

Avoid

Avoid

3

Striping with static parity

OK

OK

Avoid

Avoid

5

Striping with rotating parity

OK

Best if RAID0-1 not available

Avoid

Avoid

Table 1: RAID Recommendations (From Metalink NOTE: 45635.1)

SEE CODE DEPOT FOR FULL SCRIPTS


For more information on this topic, I recommend Don Burleson's latest book "Oracle Tuning: The Definitive Reference". 

You can buy it direct from the publisher for 50%-off and get instant access to the code depot of Oracle tuning scripts:

http://www.rampant-books.com/book_1002_oracle_tuning_definitive_reference_2nd_ed.htm

 


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