Sections “Performance During Steady State” and “Performance During Copy and Merge Operations ” describe the performance impacts on a shadow
set in steady state and while a copy or merge operation is in progress.
In general, performance during steady state compares with that of
a nonshadowed disk. Performance is affected when a copy or a merge
operation is in progress to a shadow set. In the case of copy operations,
you control when the operations are performed.
However, merge operations are not started because
of user or program actions. They are started automatically when a
system fails, or when a shadow set on a system with outstanding application
write I/O enters mount verification and times out. In this case, the
shadowing software reduces the utilization of system resources and
the effects on user activity by throttling itself dynamically. Minimerge
operations consume few resources and complete rapidly with little
or no effect on user activity.
The actual resources that are utilized during
a copy or merge operation depend on the access path to the member
units of a shadow set, which in turn depends on the way the shadow
set is configured. By far, the resources that are consumed most during
both operations are the adapter and interconnect I/O bandwidths.
control resource utilization by setting the SHADOW_MAX_COPY system
parameter to an appropriate value on a system based on the type of
system and the adapters on the machine. SHADOW_MAX_COPY is a dynamic
system parameter that controls the number of concurrent copy or merge
threads that can be active on a single system. If the number of copy
threads that start up on a particular system is more than the value
of the SHADOW_MAX_COPY parameter on that system, only the number of
threads specified by SHADOW_MAX_COPY are allowed to proceed. The other
copy threads are stalled until one of the active copy threads completes.
For example, assume that the SHADOW_MAX_COPY parameter
is set to 3. If you mount four shadow sets that all need a copy operation,
only three of the copy operations can proceed; the fourth copy operation
must wait until one of the first three operations completes. Because
copy operations use I/O bandwidth, this parameter provides a way to
limit the number of concurrent copy operations and avoid saturating
interconnects or adapters in the system. The value of SHADOW_MAX_COPY
can range from 0 to 200. The default value is OpenVMS version specific.
Chapter 3 explains how to set the SHADOW_MAX_COPY
parameter. Keep in mind that, once you arrive at a good value for
the parameter on a node, you should also reflect this change by editing
the MODPARAMS.DAT file so that when invoking AUTOGEN, the changed
value takes effect.
In addition to setting the SHADOW_MAX_COPY parameter,
the following list provides some general guidelines to control resource
utilization and the effects on system performance when shadow sets
are in transient states.
Create or add members
to shadow sets when your system is lightly loaded.
The amount of data that
a system can transfer during copy operations varies depending on the
type of disks, interconnect, controller, the number of units in the
shadow set, and the shadow set configuration on the system. For example,
approximately 5% to 15% of the Ethernet or CI bandwidth might be consumed
for each copy operation (for disks typically configured in CI or Ethernet
When you create unassisted,
three-member shadow sets consisting of one source member and two target
devices, add both target devices at the same time in a single mount
command rather than in two separate mount commands. Adding all members
at once optimizes the copy operations by starting a single copy thread
that reads from the source member once, and performs write I/O requests
to the target members in parallel.
For satellite nodes in
a mixed-interconnect or local area OpenVMS Cluster system, set the
system parameter SHADOW_MAX_COPY to a value of 0 for nodes that do
not have local disks as shadow set members.
Do not use the MOUNT/CLUSTER
command to mount every shadow set across the
cluster unless all nodes must access the set. Instead, use the MOUNT/SYSTEM
command to mount the shadow sets on only those nodes that need to
access a particular set. This reduces the chances of a shadow set
going into a merge state. Because a shadow set goes into a merge state
only when a node that has the set mounted fails, you can reduce the
chances of this happening by limiting the number of nodes that mount
a shadow set, especially when there is no need for a node to access
the shadow sets.
Because a copy operation
can occur only on nodes that have the shadow set mounted, create and
mount shadow sets on the nodes that are local (have direct access)
to the shadow set members. This allows the copy threads to run on
these nodes, resulting in faster copy operations with fewer resources
If you have shadow sets
configured across nodes that are accessed through the MSCP server,
you might need to increase the value of the MSCP_BUFFER system parameter
in order to avoid fragmentation of application I/O. Be aware that each shadow set copy or merge operation normally consumes
Dual-pathed and dual-ported
shadowed disks in a OpenVMS Cluster system can provide additional
coverage against the failure of nodes that are directly connected
to shadowed disks. This type of configuration provides higher data
availability with reasonable performance characteristics.
Use the preferred path
option to ensure dual-ported drives are accessed through the same
controller so that the shadowing software performs assisted copy operations.