What Is Minicopy?
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A minicopy operation is a streamlined copy operation. A bitmap tracks writes to a shadow set and is used to direct a minicopy operation when a shadow set member is returned to the shadow set. Instead of copying the entire contents of a device, only the changed blocks, identified by the bitmap, are copied. Minicopy ensures that the data on a shadow set member, when returned to the shadow set, is identical to the data in the shadow set.
Prior to the removal of a shadow set member, application writes are sent directly to the shadow set (also known as the virtual unit), as shown in Figure 7-1.
If you specify the minicopy qualifier (/POLICY=MINICOPY[=OPTIONAL]) when you dismount a shadow set member, a bitmap is created. Subsequent writes to the shadow set are recorded by the bitmap. Note that the bitmap records only the logical block numbers (LBNs) of the associated writes, not the contents. The address is noted by setting one or more bits in a bitmap; each bit corresponds to a range of 127 disk blocks.
When data is written to any block in the range of 127 blocks, the bit in the bitmap that corresponds to that range is set. After the bit or bits are set, the data is written to the shadow set, as shown in Figure 7-2.
When the member is returned to the shadow set, the bitmap is used to direct the minicopy operation, as shown in Figure 7-3. While the minicopy operation is taking place, the application continues to read and write to the shadow set.
With the minicopy function, a full copy is no longer required when a member is returned to its shadow set, provided that the system managers follow the guidelines provided in “Guidelines for Using a Shadow Set Member for Backup”. Note that, in this chapter, copy and full copy mean the same thing.
Several DCL commands can be used to manage bitmaps. System parameters are provided for managing the bitmap updates in an OpenVMS Cluster system and for setting an upper limit on shadow sets per node.