HP Volume Shadowing for OpenVMS |
Introduction to Volume Shadowing for OpenVMS |
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There are no restrictions on the location of shadow set members beyond the valid disk configurations defined in the SPDs for the OpenVMS operating system and for OpenVMS Cluster systems:
| For the OpenVMS Operating System: SPD 25.01.xx |
| For OpenVMS Cluster Software: SPD 29.78.xx |
If an individual disk volume is already mounted as a member of an active shadow set, the disk volume cannot be mounted as a standalone disk on another node.
Maximum
Number of Shadow Sets ![]()
You
can mount a maximum of 500 disks in two- or three-member shadow
sets on a standalone system or in an OpenVMS Cluster system. A limit
of 10,000 single member shadow sets is allowed on a standalone system
or on an OpenVMS Cluster system. Dismounted shadow sets, unused
shadow sets, and shadow sets with no write bitmaps allocated to
them are included in this total. These limits are independent of
controller and disk type. The shadow sets can be mounted as public
or private volumes.
Starting with OpenVMS Version 7.3, the SHADOW_MAX_UNIT system parameter is available for specifying the maximum number of shadow sets that can exist on a node. For more information about SHADOW_MAX_UNIT, see Volume Shadowing Parameters and Guidelines for Using Volume Shadowing Parameters.
Shadowing System Disks ![]()
You can shadow system disks as well as data disks. Thus, a
system disk need not be a single point of failure for any system
that boots from that disk. System disk shadowing becomes especially
important for OpenVMS Cluster systems that use a common system disk
from which multiple computers boot. Volume shadowing makes
use of the OpenVMS distributed lock manager, and the quorum disk
must be accessed before locking is enabled. Note that you cannot
shadow quorum disks.
Alpha and VAX systems can share data on shadowed data disks, but separate system disks are required --- one for Alpha systems and one for VAX systems.
Obtaining
Dump Files of Shadowed System Disk When Minicopy Is Used ![]()
If
you use a minicopy operation to return a member to the shadow set
and you are running OpenVMS Alpha Version 7.2-2 or Version
7.3, you must perform additional steps to access the dump file (SYSDUMP.DMP) from
a system disk shadow set. This section describes these steps.
Starting with OpenVMS Alpha Version 7.3-1, this procedure is not required because of the /SHADOW_MEMBER qualifier that was introduced for the System Dump Analyzer (SDA). SDA (referenced in step 2) is the OpenVMS utility for analyzing dump files and is documented in the OpenVMS System Analysis Tools Manual.
When the primitive file system writes a crash dump, the writes are not recorded in the write bitmap data structure. Therefore, perform the following steps:
$ SET DEVICE/READ_COST=nnn $allo_class$ddcuBy setting the read cost to a low value on that member, any reads done by SDA or by the SDA command COPY are directed to that member. HP recommends setting /READ_COST to 1.
$ SET DEVICE/READ_COST=0 DSAnnnn
Using Minicopy in a Mixed-Version OpenVMS
Cluster System ![]()
To use the minicopy feature in a mixed-version OpenVMS Cluster
system, every node in the cluster must use a
version of OpenVMS that contains this feature. Minicopy is supported
on OpenVMS Alpha Version 7.2-2, OpenVMS Alpha Version 7.3, and OpenVMS
Alpha Version 7.3-1. OpenVMS VAX Version 7.3 provides limited
support.
Using Minicopy in a Mixed-Architecture OpenVMS
Cluster System ![]()
If you intend to use the minicopy feature in a mixed-architecture
OpenVMS Cluster system, HP advises you to set the SHADOW_MAX_COPY
system parameter to zero on all VAX systems. This setting prevents
a copy from being performed on a VAX when the intent was to perform
a minicopy on an Alpha. In a mixed-architecture cluster, it is possible,
although highly unlikely, that a VAX system could be assigned the task
of adding a member to a shadow set. Because a VAX system cannot
perform a minicopy, it would perform a full copy instead. For more
information about SHADOW_MAX_COPY, see
Volume Shadowing Parameters.
Shadow Sets, Bound Volume Sets, and Stripe
Sets ![]()
Shadow sets also can
be constituents of a bound volume set or a stripe set. A bound volume
set consists of one or more disk volumes that have been bound into
a volume set by specifying the /BIND qualifier with the MOUNT command.
Shadowing Disks Across an OpenVMS Cluster System describes shadowing across
OpenVMS Cluster systems.
Striping (RAID) Implementation contains more information about striping and how RAID (redundant
arrays of independent disks) technology relates to volume shadowing.
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