High availability in case of disk failure

The primary methods used to protect against disk failure involve logical volume configuration settings, such as mirroring.

While the volume group considerations are secondary, they have significant economic implications because they involve the number of physical volumes per volume group:

• The quorum configuration, which is the default, keeps the volume group active (varied on) as long as a quorum (51%) of the disks is present. In most cases, you need at least three disks with mirrored copies in the volume group to protect against disk failure.
• The nonquorum configuration keeps the volume group active (varied on) as long as one VGDA is available on a disk. With this configuration, you need only two disks with mirrored copies in the volume group to protect against disk failure.

When deciding on the number of disks in each volume group, you must also plan for room to mirror the data. Keep in mind that you can only mirror and move data between disks that are in the same volume group. If the site uses large file systems, finding disk space on which to mirror could become a problem at a later time. Be aware of the implications on availability of inter-disk settings for logical volume copies and intra-disk allocation for a logical volume.