A minicopy operation
is a streamlined copy operation. 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.
A write 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.
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
Application Writes to a Shadow Set.
Figure 1 Application Writes to a Shadow Set
If you specify
the minicopy qualifier (/POLICY=MINICOPY[=OPTIONAL]) when you dismount
a shadow set member, a write bitmap is created. Subsequent writes
to the shadow set are recorded by the write bitmap. Note that the
write 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 write 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 write 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
Application Writes to a Write Bitmap.
Figure 2 Application Writes to a Write Bitmap
When the member is returned to the shadow set, the write bitmap
is used to direct the minicopy operation, as shown in
Member Returned to the Shadow Set (Virtual Unit). While the minicopy operation
is taking place, the application continues to read and write to
the shadow set.
Figure 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 write bitmaps.
System parameters are provided for managing the write bitmap updates
in an OpenVMS Cluster system and for setting an upper limit on shadow
sets per node.
HP Volume Shadowing for OpenVMS
Using Minicopy for Backing Up Data (Alpha)
