IBM SAS RAID controllers, Manual

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If one or two disks fail in a RAID 6 array, you can continue to use the array normally. A RAID 6 array
that is operating with a one or two failed disks is said to be operating in degraded mode. Whenever data
is read from a degraded disk array, the array controller recalculates the data on the failed disks by using
data and parity blocks on the operational disks. A RAID 6 array with a single failed disk has similar
protection to that of a RAID 5 array with no disk failures. If a third disk fails, the array is placed in the
failed state and is not be accessible.
Related information :
RAID 6 concepts
System mirroring:
Mirrored protection is beneficial if you have a multibus system or a system with a large single bus. A
greater number of disk units provides more opportunity for failure and increased recovery time.
Refer to Mirrored protection for more information.
Disk array capacities
These guidelines help you calculate the capacity of a disk array.
The capacity of a disk array depends on the capacity of the disks that are used and the RAID level of the
array. To calculate the capacity of a disk array, use the following methods:
RAID 5
Multiply one fewer than the number of disks by the disk capacity.
RAID 6
Multiply two fewer than the number of disks by the disk capacity.
System mirroring
Multiply the number of disks by the disk capacity and divide by two.
Note: If disks of different capacities are used in the same disk array, all disks are treated as if they have
the capacity of the smallest disk.
RAID level summary
Compare RAID levels according to their capabilities.
Figure 21. RAID 6
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SAS RAID controllers for IBM i