10.2. How drive linking is implemented

The drive link plug-in consumes storage objects, called link objects, which produce a larger drive link object whose address space spans the link objects. The drive link plug-in knows how to assemble the link objects so as to create the exact same address space every time. The information required to do this is kept on each link child as persistent drive-link metadata. During discovery, the drive link plug-in inspects each known storage object for this metadata. The presence of this metadata identifies the storage object as a link object. The information contained in the metadata is sufficient to:

• Identify the link object itself.

• Identify the drive link storage object that the link object belongs to.

• Identify all link objects belonging to the drive link storage. object

• Establish the order in which to combine the child link objects.

If any link objects are missing at the conclusion of the discovery process, the drive link storage object contains gaps where the missing link objects occur. In such cases, the drive link plug-in attempts to fill in the gap with a substitute link object and construct the drive link storage object in read-only mode, which allows for recovery action. The missing object might reside on removable storage that has been removed or perhaps a lower layer plug-in failed to produce the missing object. Whatever the reason, a read-only drive link storage object, together logging errors, help you take the appropriate actions to recover the drive link.