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Servers - RAID level descriptions
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RAID Level 0

Characteristics
Data is striped across all drives
Drive spindles need not be synchronised
Number of drives and performance is scalable

Error Recovery
No parity information stored on the drives
Data recovery is limited to the drive electronics
The only way to recover the data is to use system backups

Advantages
High I/O transaction rate
Multiple simultaneous seeks improve speed
Transparent to system software
Faster in smaller server applications
Load is balanced across all drives

Disadvantages
No parity or redundancy
No increase in data availability

RAID Level 1

Error Recovery
Error recovery is through the mirrored drive

Characteristics
Data duplicated on mirrored drives
Drive spindles are not synchronised
Performance is a function of the number of drives
Load is not balanced across the drives

Advantages
Maximum data availability
Transparent to system software
No performance penalty after a drive failure

Disadvantages
100% cost overhead
Requires twice the space and power

RAID Level 2

Error Recovery
Errors are corrected by recalculating the failed bit from the ECC data stored in the array

Characteristics
Data is spread across all the drives
Drive spindles must be synchrinised
The larger the array, the more efficient the storage capacity of the array
Data is corrected by the controller on the fly

Advantages
High data transfer rate
High data security, as data on any single dive is useless
High data availability
Transparent to system software

Disadvantages
Poor for transaction processing
High cost for small arrays
Special drives/drivers/operating systems required

RAID Level 3

Error Recovery
Error recovery is through the use of parity information stored on dedicated drive
Performed automatically by the disk controller

Characteristics
Data is spread acroos all the drives in bytes
Drive spindles must be synchronised
Number of drives and performance is scalable
Suited for large file transfers
Suitable for workstations that require fast sequential access to a single large file

Advantages
Highest data transfer rate of any RAID level
Increased reliability and data availability

Disadvantages:
Poor performance for small block transfers
Slower than RAID-5 for transaction processing
Unsuitable for a multi-tasking system
May create integration problems [Logical block write size = (number of drives) * (drive sector size)

RAID Level 4

Error Recovery
Error recovery is through the use of parity information stored on dedicated drive
Done automatically

Characteristics

Data is spread across all the drives in blocks
Drive spindles are not synchronised
Dedicated parity drive
Independant actuators
Number of drives and performance is scalable

Advantages
Good performance for reads during transaction processing
Multiple actuator seeks improve speed
Increased reliability and data availability

Disadvantages
Poor performance for write operations
Slower than RAID-5 for transaction processing
Parity disk bottleneck
Performance degradation during data reconstruction

RAID Level 5

Error Recovery
Error recovery is through the use of parity information distributed across multiple drives
Done automatically by the disk controller

Characteristics
Data is spread across all the drives in blocks
Drive spindles are not synchronized
Parity is spread across all drives
Independant actuators

Advantages
Good performance for all transaction processing systems
Multiple actuator seeks improve speed
Increased reliability and data availability
Transparent to system software
Increased reliability with minimal drive cost

Disadvantages
Performance degradation during data reconstruction

RAID Level 6

Error Recovery
Error recovery is through the use of mirrored drives

Characteristics
Data is spread and striped across all the drives in blocks
Data is duplicated across two SCSI buses
Drive spindles are not synchronised
Number of drives and performance is scalable

Advantages
Highest I/O transaction rateof any RAID level
Multiple actuator seeks improve speed
Maximum data availability
Transparent to system software
No performance penalty after a drive failure
No performance penalty in high write environments

Disadvantages
An instant 100% cost overhead
Twice the space and power requirements
Document ID:  CPES-38LFRJ
Last Modified:  2000-07-10
Copyright 2000 IBM Corporation
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