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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 | | |
