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HP Volume Shadowing for OpenVMS
SPD 27.29.21
Volume Shadowing for OpenVMS implements a host-
Shadow Set Membership
based approach to shadowing disk devices. In previ-
ous OpenVMS versions, Volume Shadowing for Open-
A shadow set can have one, two, or three members.
VMS VAX implemented a controller-based approach to
If one member of a two-member or a three-member
disk shadowing. Controller-based Volume Shadowing
shadow set fails, the shadow set can continue opera-
for OpenVMS VAX is no longer available or supported.
tion with the remaining full member(s).
For the purposes of this document, the term system
Shadow set members can be added to or removed from
refers to a single Integrity server, Alpha or VAX com-
the shadow set without affecting system or user opera-
puter that contains one or more processors. A system
tion, providing that one full member exists. A disk can be
can be a standalone system or a member of an Open-
removed from the shadow set either by operator com-
VMS Cluster system. The term OpenVMS Cluster refers
mand or automatically by the shadowing software. An
to a cluster that may contain a mix of Alpha systems and
inoperative disk is removed from the shadow set auto-
VAX systems or Integry server systems and Alpha sys-
matically; operator intervention is not required.
tems configured together.
Adding a disk to a shadow set requires an explicit op-
Volume Shadowing for OpenVMS supports clusterwide
erator command. The shadowing software ensures that
shadowing of Alpha SCSI, DSA, and Fibre Channel disk
data on the newly added disk is made consistent with
storage systems. Specifically, this implementation sup-
the other members by means of a copy operation. Full
ports:
read and write access to the shadow set continues while
the copy operation is in progress to the copy member.
•
HSC, HSD, HSF, HSG, HSJ, HSV, HSZ, MSA, XP,
and K.SCSI controlled disks
Transparency to the End User
•
All DSA, Fibre Channel, and Alpha SCSI adapters
To users and application programs, Volume Shadow-
and controllers that are locally connected to a system
ing for OpenVMS transparently combines the physical
members of a given shadow set into a single virtual unit.
•
RF-series controllers and disks connected to the Dig-
The shadow set virtual unit acts as a single disk. User
ital Storage Systems Interconnect (DSSI)
or application program modifications are not necessary
Volume Shadowing also supports OpenVMS MSCP
to propagate write data to shadow set members; Vol-
served DSA, Fibre Channel, and Alpha SCSI disks lo-
ume Shadowing for OpenVMS software automatically
cated within any supported OpenVMS Cluster configura-
propagates the data to all shadow set members. Simi-
tion. HP DECram virtual disks can be volume shadowed
larly, user and application read operations to the virtual
only with other DECram virtual disks.
disk are transparently routed to the optimal shadow set
member.
Volume Shadowing for OpenVMS allows shadowing of
disks that are accessible from the system on which the
Volume Shadowing for OpenVMS is invisible to applica-
shadowing software is installed. An OpenVMS system
tion programs and users. All commands and program-
parameter enables shadowing at system initialization.
ming language features that address data on nonshad-
owed disks can be used unchanged to address data on
Configuration Overview
a virtual unit.
Volume Shadowing for OpenVMS requires a minimum
Minicopy and Backup
of one system, a disk controller, and a disk unit that is
DSA compliant, Fibre Channel compliant, or SCSI com-
The minicopy operation, introduced in Volume Shadow-
pliant. Although only one disk is required for a shadow
ing on OpenVMS Alpha Versions 7.2-2 and 7.3, is a
set, two or more disks are required to maintain multiple
streamlined copy operation. Minicopy is designed to
copies of the same data. This protects against failure
be used in place of a copy operation when a former
or deterioration of a single device.
shadow set member is returned to a shadow set. When
a full member has been removed from a shadow set (for
Using multiple controllers provides a further guarantee
example, to back up that shadow set), a write bitmap
of data availability in the event that a single controller
tracks the changes that are made to the shadow set in
fails. OpenVMS Cluster systems can be configured with
the member’s absence. With minicopy, you no longer
multiple systems, interconnects, controllers, and disks;
need to dismount the entire virtual unit (shadow set) to
the resulting configurations can provide extremely high
back up the data. Stopping application write I/O, prior to
data availability.
dismounting a full member using certain minicopy quali-
fiers, is the responsibility of the user. When the member
is returned to the shadow set, the write bitmap is used
to direct the minicopy operation. While the minicopy op-
eration takes place, the application can continue to read
and write to the shadow set.
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