Compaq 108164-003 - ProLiant - 800 Technical Manual page 16

I/O Performance Tuning of Compaq Servers
service. To a client, distributing the workload means that any individual request will be filled
more quickly. By spanning the data and workload to multiple devices, it becomes more likely that
an individual drive will be able to handle a request immediately. In effect, RAID allows parallel
retrieval of data. With RAID more requests can be filled simultaneously than with a single drive.
Perhaps the greatest benefit that can be gained from using RAID technology is data guarding.
RAID systems can provide highly reliable redundancy for storage systems. In some cases RAID
can even reconstruct data that is on a drive that has failed. Proper implementation of RAID,
however, requires a basic knowledge of the systems and options involved.
RAID 0 (
No Fault Tolerance):
not provide data redundancy; therefore, provides no fault protection against data loss. RAID 0 is
known as "stripe sets" because data is simply striped across all of the drives in the array. This
configuration provides high performance at a low cost. However, you incur a risk of possible data
loss. You may consider assigning RAID level 0 to drives that require large capacity (in some
cases, full capacity of the disks) and high speed, and can afford the loss of data in the event of a
disk failure.
RAID 1 (
Disk Mirroring):
storage capacity to provide greater data reliability by storing a duplicate of all user data on a
separate disk drive. Therefore, half of the drives in the array are duplicated or "mirrored" by the
other half. This RAID level provides high level of fault tolerance, but your drive cost doubles
because this level requires twice as many disk drives to store the same amount of data and
therefore might not be cost-effective for your environment.
RAID 4
(Data Guarding):
calculate the parity, data from all the data drives in the stripe set are read. RAID 4 is not
commonly used for database applications.
Data 1
2
Data
3
Data
4
Data
Data 1
Figure 5. Data and Parity Distribution for RAID
ECG044.0399
This RAID level is not a true fault tolerance method because it does
This configuration of mirrored sets of data uses 50 percent of drive
In RAID 4 one of the disks in the stripe set is used for drive parity. To
Data 1 Data 1
2 2
Data Data
3 3
Data Data
4 4
Data Data
RAID 0 - Data Striping
Data 1 Data 1
Copy
Data 2 Data 2
Data 2
of
1
Data
RAID 1 - Drive Mirroring
Data 1
Data 1
2
2
Data
Data
3
3
Data
Data
4
4
Data
Data
Data 1
Parity 1
Copy
Parity 2
Data 2
of
2
Data
Data 3
4
Data
RAID 5 - Distributed Data Guarding
Data 1 Data 1
2 2
Data Data
3 3
Data Data
4 4
Data Data
RAID 4 - Data Guarding
Data 1 Data 1
Data 2 Data 2
Parity 3
Data 3
4
Parity 4
Data
16
Parity 1
Parity 2
Parity 3
Parity 4
Parity 1
Data 2
Data 3
4
Data