Novell LINUX ENTERPRISE SERVER 10 - STORAGE ADMINISTRATION GUIDE FOR EVMS, Administration Manual

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Managing Software RAIDs 6 and 10 with mdadm
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Managing Software RAIDs 6 and
10 with mdadm
This section discusses how to create software RAID 6 and 10 devices, using the Multiple Devices
Administration (
mdadm(8)
) tool. You can also use
mdadm
to create RAIDs 0, 1, 4, and 5. The
mdadm
tool provides the functionality of legacy programs
mdtools
and
raidtools
.
Š
Section 7.1, “Creating a RAID 6,” on page 75
Š Section 7.2, “Creating Nested RAID 10 Devices with mdadm,” on page 76
Š
Section 7.3, “Creating a Complex RAID 10 with mdadm,” on page 79
Š Section 7.4, “Creating a Degraded RAID Array,” on page 83
7.1 Creating a RAID 6
Š Section 7.1.1, “Understanding RAID 6,” on page 75
Š
Section 7.1.2, “Creating a RAID 6,” on page 76
7.1.1 Understanding RAID 6
RAID 6 is essentially an extension of RAID 5 that allows for additional fault tolerance by using a
second independent distributed parity scheme (dual parity). Even if one of the hard disk drives fails
during the data recovery process, the systems continues operational, with no data loss.
RAID6 provides for extremely high data fault tolerance by sustaining multiple simultaneous drive
failures. It handles the loss of any two devices without data loss. Accordingly, it requires N+2 drives
to store N drives worth of data. It requires a minimum of 4 devices.
The performance for RAID 6 is slightly lower but comparable to RAID 5 in normal mode and single
disk failure mode. It is very slow in dual disk failure mode, however.
Table 7-1
Comparison of RAID 5 and RAID 6
Feature RAID 5 RAID 6
Number of devices N+1, minimum of 3 N+2, minimum of 4
Parity Distributed, single Distributed, dual
Performance Medium impact on write and
rebuild More impact on sequential write
than RAID 5
Fault-tolerance Failure of one component device Failure of two component devices