Mirroring and Duplexing

You can protect the data mirroring or duplexing disk partitions.

Mirroring stores the same data on separate disks on the same controller channel; duplexing stores the same data on separate disks on separate controller channels. Duplexing can also use two different drivers. Duplexing is the recommended method because two channels rarely fail simultaneously.

In the following illustration, several smaller disks are mirrored to partitions on one larger disk.

To mirror partitions you must enable Hot Fix. For information about Hot Fix, see Redirecting Bad Blocks.

With Hot Fix enabled, a section of the partition is set aside as the Hot Fix/Mirror object, which holds data tables for both Hot Fix and mirroring. The tables store information about which data blocks are bad, which blocks have been redirected, which partitions are mirrored, and whether the partitions are synchronized or not.

Setting up Hot Fix is much easier when you create the partition. To add Hot Fix after you create a partition and add volumes, you must delete the volumes from the partition, add Hot Fix, then restore the volumes from a backup. When you restore volumes from a backup, make sure you have enough space on the partition to accommodate the volumes after you add Hot Fix to the data area.

The following are important concepts for mirroring partitions:

• Mirrored partitions must have the same partition type of mirror group as the group you assign it to. This means that NSS partitions can be mirrored only to other NSS partitions. The same criteria applies to traditional partitions.
• Mirrored partitions must be compatible in data area size. This means the new partition must be at least the same size or slightly larger than the other partitions in the group. The physical size (combined data and Hot Fix size) of the partition must be at least 100 KB, but no more than 120 MB larger than the data size of the existing partitions in the mirror group.(Note: The file system adjusts the Hot Fix size within the legal ranges in order to make the data area identical to the other partitions in the group.)
• Mirrored partitions must have the same sharable status.
• Partitions you add to a mirror group cannot be part of any other mirror group. They must be standalone Mirror objects.
• The partitions you add must include the Mirror and Hot Fix options. You select these options when you create the partition; you cannot add the options later.

• When you select Hot Fix, mirroring is automatically selected. You cannot mirror partitions that do not have a Hot Fix Redirection Area.
• Although you can mirror one partition to as many as eight other partitions, mirroring two partitions is usually sufficient fault tolerance for most systems.
• If a mirrored disk fails and cannot be accessed by the server, you can unmirror the hard disks and salvage the volume from the functional disk. See Recovering Data from an Out of Sync Disk.
• If you want to remove a hot-plug mirrored disk without bringing down the server, you must unmirror the disk first. See Unmirroring Partitions.

For information about how to set up mirroring and duplexing, see Mirroring and Duplexing Partitions.