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.
Figure 7-1 Application Writes to a Shadow Set
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.
Figure 7-2 Application Writes to a Bitmap
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.
Figure 7-3 Member Returned to the Shadow Set (Virtual Unit)
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.