Promise Technology E310f, Product Manual

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Chapter 7: Technology Background
239
Failover and Failback
When one controller fails, the surviving controller takes over logical drive access
until the failed controller is brought back online or is replaced. For example, LUN
Affinity is enabled and your logical drives are assigned to Controller 1. The
following actions will happen:
• If Controller 1 goes offline, Controller 2 takes over access to the logical
drives assigned to Controller 1.
• If Controller 1 comes back online, Controller 1 takes back access to the
logical drives assigned to it.
• If Controller 1 is replaced, the new controller takes over access to the logical
drives assigned to Controller 1.
• All logical drives assigned to Controller 2 remain accessible by Controller 2.
Controller 1 cannot access them at any time.
Capacity Coercion
This feature is designed for fault-tolerant logical drives (RAID 1, 1E, 5, 10, 50,
and 60). It is generally recommended to use physical drives of the same size in
your disk arrays. When this is not possible, physical drives of different sizes will
work but the system must adjust for the size differences by reducing or coercing
the capacity of the larger drives to match the smaller ones. With VTrak, you can
choose to enable Capacity Coercion and any one of four methods.
Enable Capacity Coercion and select the Method in the Controller Settings menu.
See page 101. The choices are:
• GB Truncate – (Default) Reduces the useful capacity to the nearest
1,000,000,000 byte boundary.
• 10GB Truncate – Reduces the useful capacity to the nearest 10,000,000,000
byte boundary.
• Group Rounding – Uses an algorithm to determine how much to truncate.
Results in the maximum amount of usable drive capacity.
• Table Rounding – Applies a predefined table to determine how much to
truncate.
Capacity Coercion also affects a replacement drive used in a disk array.
Normally, when an physical drive fails, the replacement drive must be the same
capacity or larger. However, the Capacity Coercion feature permits the
installation of a replacement drive that is slightly smaller (within 1 gigabyte) than
the remaining working drive. For example, the remaining working drives can be
80.5 GB and the replacement drive can be 80.3, since all are rounded down to
80 GB. This permits the smaller drive to be used.