Write Bitmaps and Dissimilar Device Shadowing Caution

DDS, introduced in OpenVMS Version 7.3-2, allows you to construct shadow sets of disk devices that are of dissimilar sizes.

Write bitmaps track application writes made to a shadow set virtual unit so that a member can be returned to that virtual unit without the overhead of a full copy. A write bitmap is created when the user issues a DISMOUNT/POLICY=MINICOPY command for a shadow set member or mounts a shadow set using the MOUNT/POLICY=MINICOPY command. When this bitmap is created, its size depends on the current size of the volume.

When a shadow set is mounted, the logical size of the shadow set virtual unit is set to the size of the smallest member unit. When a member of the shadow set is removed, the logical size of the virtual unit is recomputed based on the sizes of the remaining members of the set. Consequently, the logical size of the virtual unit might increase.

When a write bitmap is created for a shadow set, its size is determined by the current size of the shadow set virtual unit. If the virtual unit’s size subsequently increases, the bitmap does not cover the entire virtual unit. If the bitmap is then used to bring back a shadow set member with a minicopy operation, the portion of the virtual unit that is not covered by the bitmap is copied with a full copy operation.

The following example illustrates this problem:

If the removal of a smaller shadow set member is planned, removing it before removing a larger member with a minicopy bitmap causes a larger bitmap to be created and avoids the performance impact of a short bitmap. (In the preceding example, you would remove $1$DGA20: before removing $1$DGA22:.)