Raid 5; Raid 6 - Lenovo ThinkServer RD650 User Manual

12Gb/s MegaRAID SAS Software User Guide
March 2014
2.2.5

RAID 5

RAID 5 includes disk striping at the block level and parity. Parity is the data's property of being odd or even, and parity
checking is used to detect errors in the data. In RAID 5, the parity information is written to all drives. RAID 5 is best
suited for networks that perform a lot of small input/output (I/O) transactions simultaneously.
RAID 5 addresses the bottleneck issue for random I/O operations. Because each drive contains both data and parity,
numerous writes can take place concurrently.
The following table provides an overview of RAID 5. The following figure provides a graphic example of a RAID 5 drive
group.
Table 9 RAID 5 Overview
Uses
Strong points
Weak points
Number of Drives in
this RAID level
Figure 9 RAID 5 Drive Group with Six Drives
2.2.6

RAID 6

RAID 6 is similar to RAID 5 (disk striping and parity), except that instead of one parity block per stripe, there are two.
With two independent parity blocks, RAID 6 can survive the loss of any two drives in a virtual drive without losing
data. RAID 6 provides a high level of data protection through the use of a second parity block in each stripe. Use
RAID 6 for data that requires a very high level of protection from loss.
In the case of a failure of one drive or two drives in a virtual drive, the RAID controller uses the parity blocks to
re-create all of the missing information. If two drives in a RAID 6 virtual drive fail, two drive rebuilds are required, one
for each drive. These rebuilds do not occur at the same time. The controller rebuilds one failed drive, and then the
other failed drive.
The following table provides an overview of a RAID 6 drive group.
Provides high data throughput, especially for large files. Use RAID 5 for transaction processing applications
because each drive can read and write independently. If a drive fails, the RAID controller uses the parity drive
to re-create all missing information. Use also for office automation and online customer service that requires
fault tolerance. Use for any application that has high read request rates but low write request rates.
Provides data redundancy, high read rates, and good performance in most environments. Provides
redundancy with lowest loss of capacity.
Not well-suited to tasks requiring lot of writes. Suffers more impact if no cache is used (clustering). Drive
performance is reduced if a drive is being rebuilt. Environments with few processes do not perform as well
because the RAID overhead is not offset by the performance gains in handling simultaneous processes.
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LSI Corporation
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Chapter 2: Introduction to RAID
RAID Levels