HP Volume Shadowing for OpenVMS

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