Maximizing Fault Tolerance - Lenovo ThinkServer RD330 Software User's Manual

MegaRAID SAS Software User Guide
2.6.1

Maximizing Fault Tolerance

Table 15: RAID Levels and Fault Tolerance
RAID
Level
0 Does not provide fault tolerance. All data is lost if any drive fails. Disk striping writes data across multiple drives instead of just one drive.
It involves partitioning each drive storage space into stripes that can vary in size. RAID 0 is ideal for applications that require high
bandwidth but do not require fault tolerance.
1 Provides complete data redundancy. If one drive fails, the contents of the other drive in the drive group can be used to run the system
and reconstruct the failed drive.
The primary advantage of disk mirroring is that it provides 100 percent data redundancy. Since the contents of the drive are completely
written to a second drive, no data is lost if one of the drives fails. Both drives contain the same data at all times. RAID 1 is ideal for any
application that requires fault tolerance and minimal capacity.
5 Combines distributed parity with disk striping. Parity provides redundancy for one drive failure without duplicating the contents of
entire drives. If a drive fails, the RAID controller uses the parity data to reconstruct all missing information. In RAID 5, this method is
applied to entire drives or stripes across all drives in a drive group. Using distributed parity, RAID 5 offers fault tolerance with limited
overhead.
6 Combines distributed parity with disk striping. RAID 6 can sustain two drive failures and still maintain data integrity. Parity provides
redundancy for two drive failures without duplicating the contents of entire drives. If a drive fails, the RAID controller uses the parity
data to reconstruct all missing information. In RAID 6, this method is applied to entire drives or stripes across all of the drives in a drive
group. Using distributed parity, RAID 6 offers fault tolerance with limited overhead.
00 Does not provide fault tolerance. All data in a virtual drive is lost if any drive in that virtual drive fails. Disk striping writes data across
multiple drives instead of just one drive. It involves partitioning each drive storage space into stripes that can vary in size. RAID 00 is
ideal for applications that require high bandwidth but do not require fault tolerance.
You cannot configure a virtual drive that optimizes all three factors, but it is easy to
choose a virtual drive configuration that maximizes one factor at the expense of
another factor. For example, RAID 1 (mirroring) provides excellent fault tolerance, but
requires a redundant drive.
The following subsections describe how to use the RAID levels to maximize virtual
drive availability (fault tolerance), virtual drive performance, and virtual drive capacity.
Fault tolerance is achieved through the ability to perform automatic and transparent
rebuilds using hot spare drives and hot swaps. A hot spare drive is an unused online
available drive that the RAID controller instantly plugs into the system when an active
drive fails. After the hot spare is automatically moved into the RAID drive group, the
failed drive is automatically rebuilt on the spare drive. The RAID drive group continues
to handle requests while the rebuild occurs.
A hot swap is the manual substitution of a replacement unit in a disk subsystem for a
defective one, where the substitution can be performed while the subsystem is
running hot swap drives. Auto-Rebuild in the WebBIOS Configuration Utility allows a
failed drive to be replaced and automatically rebuilt by "hot-swapping" the drive in the
same drive bay. The RAID drive group continues to handle requests while the rebuild
occurs, providing a high degree of fault tolerance and zero downtime.
Fault Tolerance
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Chapter 2: Introduction to RAID RAID Configuration Strategies
Page 41