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Supported Configurations   



Volume Shadowing for OpenVMS provides data availability across the full range of configurations---from single nodes to large OpenVMS Cluster systems---so you can provide data availability where you need it most.

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:

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:

  1. Check the console output at the time of the system failure to determine which device contains the system dump file.

    The console displays the device to which the crash dump was written. That shadow set member contains the only full copy of that file.
  2. Assign a low value to the member to which the dump was written by issuing the following command:
    $ SET DEVICE/READ_COST=nnn $allo_class$ddcu
    By 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.
  3. After you have analyzed or copied the system dump, you must return the read cost value of the shadow set member to the previous setting --- either the default setting assigned automatically by the volume shadowing software or the value you had previously assigned. If you do not, all read I/O is directed to the member with the READ_COST setting of 1, which can unnecessarily degrade read performance.

    To change the READ_COST setting of a shadow set member to its default value, issue the following command:
    $ 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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