When deciding which RAID level to use for your IBM Rational ClearCase VOB or View storage, what are the advantages, disadvantages and requirements to consider?
A RAID (Redundant Array of Independent Disks) system is a network subsystem that protects data integrity by using more than one disk for data storage. RAID relies on a series of configurations called LEVELS to determine how drives are connected and how data is written and retrieved from the disks.
The following section provides a description of the most common RAID configurations along with considerations to make concerning Rational ClearCase storage.
Refer to the following sources for more detailed information about specific requirements, advantages and disadvantages of each RAID level.
Level 0 -- Striped Disk Array without Fault Tolerance
ClearCase considerations: This can be one of the fastest performing disk array configurations for ClearCase operations, if multiple disk I/O channels are used. However, the dramatic impact of a drive failure can render this an unacceptable option.
Level 1 -- Mirroring and Duplexing
ClearCase considerations: If the mirroring and or duplexing is being handled by the host OS, performance degradation is inevitable. While it may not occur during the initial (lab) implementation of the configuration, it will eventually occur when put into production as users perform operations that require writes into the VOB databases. Use of a dedicated array controller, or the use of a SAN may mitigate this, but this configuration is not a good one for storing databases that are frequently modified.
Level 0+1 – A Mirror of Stripes
ClearCase considerations: From the standpoint of ClearCase performance, this can be a very high performing array configuration.
Level 2 -- Error-Correcting Coding
RAID Level 2 is an implementation of the Hamming Code ECC.
ClearCase considerations: Without commercial "in the field" implementations, assessing the impact of this storage configuration on ClearCase performance and reliability is not practical.
Level 3 -- Bit-Interleaved Parity
ClearCase considerations: The relatively low transaction rate of this array offers no real advantages over a single disk. The dedicated parity disk can become a bottleneck in a high-usage environment.
Level 4 -- Dedicated Parity Drive
RAID Level 4 uses independent data disks with a shared parity disk.
ClearCase considerations: This array configuration offers no major performance advantages over a single disk. The dedicated parity disk can become a bottleneck in a high-usage environment.
RAID Level 5 also relies on parity information to provide redundancy and fault tolerance using independent data disks with distributed parity blocks.
ClearCase considerations: Initial implementations of RAID 5 often had reliability problems in ClearCase environment, but those have been resolved. Given a properly optimized RAID controller, this can perform on a par with RAID 0 configurations.
RAID Level 6 uses independent data disks with two independent distributed parity schemes.
ClearCase considerations: Poor write performance (in comparison with other RAID levels) usually eliminates this RAID level as a candidate for use in ClearCase environments.
ClearCase considerations: Strictly speaking, this can be a fairly high-performing array configuration for ClearCase; however, the disk maintenance often makes this configuration difficult to justify the cost.
ClearCase considerations: This configuration can provide very high performance. In theory this can be faster than RAID 5, but real-world performance depends on the controller solution as handling the complexity of the configuration may degrade performance.
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