Technical data

System Management Features
4.13 Volume Shadowing for OpenVMS
4.13.1 Minicopy in Compaq Volume Shadowing for OpenVMS (Alpha)
The new minicopy feature of Compaq Volume Shadowing for OpenVMS and its
enabling technology, write bitmap, are fully implemented on OpenVMS Alpha
systems. OpenVMS VAX nodes can write to shadow sets that use this feature
but they can neither create master write bitmaps nor manage them with DCL
commands.
The minicopy operation is a streamlined copy operation. Minicopy is designed to
be used in place of a copy operation when you return a shadow set member to
the shadow set. When a member has been removed from a shadow set, a write
bitmap tracks the changes that are made to the shadow set in its absence, as
shown in Figure 4–1.
Figure 4–1 Application Writes to a Write Bitmap
Disk 2
removed
from VU
Disk 1
Write Bitmap
Back up
to tape
Application writes
Active Virtual Unit
VM-0473A-AI
When the member is returned to the shadow set, the write bitmap is used to
direct the minicopy operation, as shown in Figure 4–2. While the minicopy
operation is taking place, the application continues to read and write to the
shadow set.
Figure 4–2 Member Returned to the Shadow Set (Virtual Unit)
Application writes
Disk 2
returned
to VU
Disk 1
Only blocks that
have been changed
are copied
Active Virtual Unit
Write Bitmap
VM-0474A-AI
Thus, minicopy can significantly decrease the time it takes to return the member
to membership in the shadow set and can significantly increase the availability of
the shadow sets that use this feature.
Typically, a shadow set member is removed from a shadow set to back up the
data on the disk. Before the introduction of the minicopy feature, Compaq
required that the virtual unit (the shadow set) be dismounted to back up the
data from one of the members. This requirement has been removed, provided
that the guidelines for removing a shadow set member for backup purposes, as
documented in Volume Shadowing for OpenVMS, are followed.
4–20 System Management Features