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Managing Software RAIDs 6 and
7
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.
Section 7.1, "Creating a RAID 6," on page 75
Section 7.2, "Creating Nested RAID 10 Devices with mdadm," on page 76
Section 7.3, "Creating a Complex RAID 10 with mdadm," on page 79
Section 7.4, "Creating a Degraded RAID Array," on page 83
7.1 Creating a RAID 6
Section 7.1.1, "Understanding RAID 6," on page 75
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.
Comparison of RAID 5 and RAID 6
Table 7-1
Feature
Number of devices
Parity
Performance
Fault-tolerance
RAID 5
N+1, minimum of 3
Distributed, single
Medium impact on write and
rebuild
Failure of one component device
RAID 6
N+2, minimum of 4
Distributed, dual
More impact on sequential write
than RAID 5
Failure of two component devices
Managing Software RAIDs 6 and 10 with mdadm
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75
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