HP Volume Shadowing for OpenVMS

HP Volume Shadowing for OpenVMS SPD 27.29.21
The minicopy feature and its enabling technology, write
bitmaps, are fully implemented for OpenVMS Alpha and
OpenVMS for Integrity servers. OpenVMS VAX V7.3
systems can write to shadow sets which use this fea-
ture, but they can neither create master write bitmaps
nor manage them with DCL commands. In a mixed-
version cluster if any systems are running a version of
OpenVMS that does not have minicopy capability, no
system in the cluster can use the minicopy feature.
Host-Based Minimerge
In a full merge operation, the members of a shadow
set are compared with each other to ensure that they
contain the same data. This is done by performing a
block-by-block comparison of the entire volume. This
can be a very lengthy procedure.
A minimerge operation can be significantly faster. By us-
ing information about write operations that were logged
in volatile controller storage or in a write bitmap on
an OpenVMS system, volume shadowing merges only
those areas of the shadow set where write activity oc-
curred. This avoids the need for the entire volume
scan that is required by full merge operations, thus
reducing consumption of system I/O resources. In a
mixed-version cluster, every system must support write
bitmaps, but it isn’t necessary that every system sup-
port host-based minimerge (HBMM). Only systems that
support HBMM can mount HBMM enabled shadow sets.
Host-based minimerge (HBMM), introduced in Volume
Shadowing on OpenVMS V8.2, enables OpenVMS
systems to track specific write operations using write
bitmaps. HBMM can be used on all storage types, with
the exception of those types that have been enabled for
controller-based minimerge.
For more information on Host-based minimerge, refer
to the HBMM chapter in the HP OpenVMS Version 8.2
New Features and Documentation Overview manual.
Multiple-Site Clusters
Starting with OpenVMS Alpha Version 7.2-2 and avail-
able on OpenVMS for Integrity server systems, new
qualifiers for the DCL commands DISMOUNT and SET
were introduced for use in multiple-site cluster config-
urations to provide disaster-tolerant support. Designed
primarily for multiple-site clusters that use Fibre Channel
as a site-to-site interconnect, these command qualifiers
can be used in other configurations as well. For more
details, refer to the HP Volume Shadowing for Open-
VMS manual.
Compatibility
HP Volume Shadowing for OpenVMS Alpha and HP Vol-
ume Shadowing for Integrity servers are compatible and
can operate in an OpenVMS Cluster system, provided
that the same features are enabled on both systems.
The same is true for HP Volume Shadowing for Open-
VMS Alpha and HP Volume Shadowing for OpenVMS
VAX. The use of both products in a mixed-architecture
OpenVMS Cluster (Alpha and VAX machines clustered
together, or Alpha and Integrity server machines clus-
tered together) can enable the sharing of data disks.
Configuration Limits
Volume Shadowing for OpenVMS supports a maxi-
mum of 500 disks in multiple-member (2- or 3-member)
shadow sets on a single OpenVMS Cluster. Up to
10,000 single-member shadow sets are allowed on the
same OpenVMS Cluster or standalone system. These
limits are independent of controller and device type.
Configuration Restrictions
Controller-based Volume Shadowing for OpenVMS VAX
is no longer available.
In Volume Shadowing for OpenVMS VAX V7.3 and Vol-
ume Shadowing for OpenVMS Alpha Version 7.3-1, and
all earlier versions, all members of a shadow set must
have the same number of physical blocks. For example,
two RA92 disk drives could form a shadow set. An RZ28
and an RZ28B disk drive could also form a shadow set.
However, a 6-GB disk and an 8-GB disk could not be
members of the same shadow set.
Starting with OpenVMS Alpha Version 7.3–2, differ-
ent sized devices can be used within a shadow set.
This functionality is called dissimilar device shadowing
(DDS). To use DDS, all systems must be running Open-
VMS Alpha Version 7.3–2 or later.
Shadow set members cannot have hardware write pro-
tection enabled. Hardware write protection stops vol-
ume shadowing software from maintaining identical vol-
umes.
Volume Shadowing for OpenVMS does not support
shadow sets mounted with the /FOREIGN qualifier.
Volume Shadowing for OpenVMS provides support for
SCSI disks and controllers on Alpha systems. Shad-
owing can also be used with third-party SCSI disks that
have READL (read long) and WRITEL (write long) com-
mands implemented and that use the OpenVMS SCSI
disk driver. Features are restricted when SCSI disks that
do not support READL and WRITEL are shadowed; disk
bad-block errors resulting from the use of these disks
can cause members to be removed from the shadow
set.
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