 






          Software
          Product
          Description

          ________________________________________________________________

          PRODUCT NAME:  VMS Volume Shadowing, Version 5.4    SPD 27.29.07

          DESCRIPTION

          VMS Volume Shadowing is a System Integrated Product (SIP) avail-
          able on VMS systems. VMS Volume Shadowing provides high data
          availability by ensuring against data loss resulting from me-
          dia deterioration or through controller or device failure. This
          prevents storage subsystem component failures from interrupting
          system or application operation.

          Volume shadowing maintains multiple redundant copies of data on
          a collection of disks called a shadow set. Shadow sets consist
          of one, two, or three compatible disk volumes which are referred
          to as shadow set members. This duplication of data provides
          greater data availability. When data is recorded on more than
          one disk volume, the data is accessible even when one volume is
          unavailable. Disk input/output operations continue transparently
          with the remaining members of the shadow set.

          Because a shadow set is made up of multiple disks containing
          the same data, the VMS Volume Shadowing software can read from
          either disk device. The VMS Volume Shadowing software determines
          which disk to read from, per read operation.

          If sections of a volume become unreadable, systems with volume
          shadowing can read the data from the duplicate and attempt to
          repair the original by writing the data to good blocks.

          VMS system disks and any Files-11 On-Disk Structure 2 (ODS2)
          data disks can be volume shadowed.


                                       DIGITAL              September 1990

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          For the purposes of this document, the term CPU is used to refer
          to a single VAX computer. A CPU may be standalone or a member of
          a VAXcluster.

          VMS Volume Shadowing provides two modes of performing shadowing
          operations on the VMS Operating System:

          o  VAX Volume Shadowing Phase I

             This is the original implementation of the VAX Volume Shad-
             owing product. Phase I VMS Volume Shadowing provides for
             centralized shadowing on VMS systems using Hierarchical Stor-
             age Controllers (HSC). This mode of shadowing is limited to
             disks connected to HSCs, either in a VAXcluster or single
             CPU.

          o  VMS Volume Shadowing Phase II

             Phase II allows shadowing on the same configurations as Phase
             I plus it supports VAXcluster wide shadowing of disks on a
             single CPU or located anywhere in a VAXcluster system.

             VMS Volume Shadowing Phase II supports VAXcluster wide shad-
             owing of all Digital Storage Architecture (DSA) devices.
             Phase II is not limited to HSC-controlled disks, but extends
             volume shadowing capabilities to all DSA disks including
             those on local adapters, all Digital Small Systems Interco-
             nenct (the DSSI RF series) disk devices on any VAX computer,
             and VMS MSCP served disks located anywhere in any supported
             VAXcluster configuration.

          It is possible to use both modes of shadowing on the same CPU or
          VAXcluster system at the same time. Any given shadow set must be
          either Phase I or Phase II; a shadow set cannot be a mixture of
          the two modes of shadowing.

          Characteristics

          CPUs and Controllers

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          VMS Volume Shadowing requires a minimum of one VAX CPU, one
          MSCP- compliant mass storage controller, and one Digital Storage
          Architecture (DSA) disk drive and volume. Although only one disk
          is required, there must be two or more disk drives and volumes
          in order to "shadow" or maintain multiple copies of the same
          data. This configuration provides protection against either
          failure of a single disk drive or deterioration of a single
          volume.

          Using two controllers provides a further guarantee of data
          availability in the event of a single controller failure. Using
          a VAXcluster and multiple controllers provides the greatest data
          availability.

          Shadow Set Membership

          A shadow set can have one, two, or three members. The ability
          to have a shadow set with only a single member allows a two-
          member shadow set to continue operation after the failure of one
          member. It also allows a second member to be added at any time
          with no disruption of the application.

          All members of a shadow set must be of the same disk type, for
          example, all RA90s or RF31s. See the Supported Hardware Devices
          section for a list of those disk devices supported in shadow
          sets.

          Transparency to the End User

          The shadow set acts as a single disk. The user makes no effort
          to have the data propagated to all of the members of a shadow
          set. It is a task of the volume shadowing software to replicate
          the data on all shadow set members.

          VMS Volume Shadowing is invisible to application programs and
          users. All commands and program language features that address
          data on non-shadowed disks may be used to address data on shad-
          owed disks with no changes.

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          Individual disks may be added to or removed from the shadow set
          without affecting either the end user or any application.

          Shadow Set Operation

          A disk can be removed from the shadow set by operator command
          or automatically. A failed disk is removed from the shadow set
          automatically; no operator intervention is required.

          A disk is only added to the shadow set on command. Within a
          reasonable period of time, the newly added shadow set member is
          made identical to the other member(s). Full read/write access to
          the shadow set continues while the new member is being added.

          System Disk Shadow Sets

          System disk(s), like any other Files-11 ODS2 disks, can be
          shadowed.

          Restrictions

          A quorum disk cannot be shadowed in a VAXcluster system.

          Disks cannot have hardware write protection enabled. Hardware
          write protection stops volume shadowing software from maintain-
          ing identical volumes.

          In a VAXcluster with more than one Star Coupler, Phase I shadow-
          ing is limited by the following restrictions:

          o  If a VAX is to directly access a Phase I shadow set on an
             HSC, the VAX must have a direct CI path to that HSC (be
             connected to the same Star Coupler).

          o  Phase I shadow sets may be MSCP served over the CI. To do
             this, the VAXs must be on the same Star Coupler as the HSC(s)
             with the shadow set. These VAXs can then MSCP serve the
             shadow set to other VAXs connected to other Star Couplers.
             If a VAX is accessing a Phase I shadow set via another VAX's
             MSCP Server, it cannot directly access any Phase I shadow
             set on any local HSC. Essentially, a CPU must access all its

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             Phase I shadow sets either locally or via a served path. It
             cannot do both.

          o  VAXs accessing Phase I shadow sets via an MSCP served path
             (over the CI or Ethernet) cannot add or remove members of the
             shadow set, they have data access only.

          o  Phase II shadow sets are not subject to any of the above
             restrictions involving multiple star couplers.

          Phase I supports a maximum of 16 shadowed disks per HSC or HSC
          pair (for dual pathed disks).

          Phase I supports a maximum of 8 shadow sets per HSC or HSC pair
          (for dual pathed disks).

          For additional information about restrictions, refer to the VMS
          Operating System Software Product Description (SPD 25.01.xx) and
          the VAXcluster Software Product Description (SPD 29.78.xx).

          HARDWARE REQUIREMENTS - PHASE I

          Refer to the VMS Operating System Software Product Description
          (SPD 25.01.xx) and VAXcluster Software Product Description
          (SPD 29.78.xx) for information about hardware requirements and
          supported CPUs.

          At least one HSC series controller is required.

          Shadow sets must be made of identical types of disks (for exam-
          ple, all RA90s) connected to the same HSC. If the disks are dual
          pathed between two HSCs, all members of the shadow set must be
          dual pathed between the same pair of HSCs, and both HSCs must be
          connected to the same Star Coupler.





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          HARDWARE REQUIREMENTS - PHASE II

          VMS Volume Shadowing Phase II does not depend on specific hard-
          ware in order to operate. All shadowing functions can be per-
          formed on any supported VAX CPU by the VMS Operating System.
          Shadowing capabilities are supported on all disks and con-
          trollers listed in the Supported Hardware Devices section of
          this SPD.

          All members of a shadow set must be the same type of disk (for
          example, all RA90s or all RF71s). Members of a shadow set may be
          located on a single CPU or anywhere in a VAXcluster. There are
          no requirements on the location of shadow set members beyond the
          valid disk configurations defined in the VMS Operating System
          Software Product Description (SPD 25.01.xx) and VAXcluster
          Software Product Description (SPD 29.78.xx).

          Supported Hardware Devices

          VMS Volume Shadowing supports the following devices and con-
          trollers for both Phase I and Phase II shadowing, unless other-
          wise specified. These devices may be components of Storage Array
          (SAxxx) products.

                            Disk Controllers and Adaptors

          All HSCs supported by the VMS Operating System

          Controllers Supported by Phase II Shadowing Only:

          o  KDA50

          o  KDB50

          o  KDM70

          o  KFQSA

          o  RQDX3

          o  RQDX2

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          o  UDA50

          Disks Supported by Volume Shadowing:

          o  RA60

          o  RA70

          o  RA80

          o  RA81

          o  RA82

          o  RA90

          o  RA92

          o  ESE20

          o  RF30 (Phase II only)

          o  RF31 (Phase II only)

          o  RF71 (Phase II only)

          o  RD51 when connected to RQDX controller
             (Phase II only)

          o  RD52 when connected to RQDX controller
             (Phase II only)

          o  RD53 when connected to RQDX controller
             (Phase II only)

          o  RD54 when connected to RQDX controller
             (Phase II only)

          CLUSTER ENVIRONMENT

          PHASE I and PHASE II

          VMS Volume Shadowing is fully supported when installed on any
          valid and licensed VAXcluster configuration without restric-
          tions. The  HARDWARE REQUIREMENTS section of this product's
          Software Product Description and the VMS Operating System

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          VMS Volume Shadowing, Version 5.4                   SPD 27.29.07



          Software Product Description (SPD 25.01.xx) list any special
          hardware required by this product.

          VAXcluster configurations are fully described in the VAXcluster
          Software Product Description (SPD 29.78.xx).

          If VMS V5.3 and V5.4 are both present in a VAXcluster, members
          of the VAXcluster that are running VMS V5.3 can only access
          Phase I shadow sets. Members of the VAXcluster running VMS V5.4
          can access either Phase I or Phase II shadow sets.

          SOFTWARE REQUIREMENTS

          PHASE I and PHASE II

          o  VMS Operating System V5.4

          o  HSC Software V3.9

          Refer to the VAXcluster Software Product Description (SPD
          29.78.xx) for information about rolling upgrade restrictions.

          VMS Tailoring

          PHASE I and PHASE II

          For VMS V5.x, the following VMS class is required for full
          functionality of this layered product.

          o  VMS Required Saveset

          For more information about VMS classes and tailoring, refer
          to the VMS Operating System Software Product Description (SPD
          25.01.xx).

          OPTIONAL SOFTWARE

          None

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          GROWTH CONSIDERATIONS

          The minimum hardware/software requirements for any future ver-
          sion of this product may be different from the requirements for
          the current version.

          ORDERING INFORMATION

          Software Licenses: QL-AB2A*-**
          Software Documentation: QA-AB2AA-GZ
          Software Product Services: QT-AB2A*-**

          *  Denotes variant fields. For additional information on avail-
             able licenses, services and media, refer to the appropriate
             price book.

          The above information is valid at the time of release. Please
          contact your local Digital office for the most up-to-date infor-
          mation.

          SOFTWARE LICENSING

          This software is furnished under the licensing provisions of
          Digital Equipment Corporation's Standard Terms and Conditions.
          For more information about Digital's licensing terms and poli-
          cies, contact your local Digital office.

          LICENSE MANAGEMENT FACILITY

          VMS Volume Shadowing supports the VMS License Management Facil-
          ity ( LMF).

          License units for this product are allocated on a CPU-capacity
          basis.

          Every CPU in a VAXcluster that mounts a shadow set must have a
          VMS Volume Shadowing License installed.


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          For more information about the License Management Facility,
          refer to the VMS Operating System Software Product Description
          (SPD 25.01.xx) or the License Management Utility Manual of the
          VMS Operating System documentation set.

          For more information about Digital's licensing terms and poli-
          cies, contact your local Digital office.

          SOFTWARE PRODUCT SERVICES

          A variety of service options are available from Digital. For
          more information, contact your local Digital office.

          SOFTWARE WARRANTY

          Warranty for this software product is provided by Digital with
          the purchase of a license for the product as defined in the
          Software Warranty Addendum of this SPD.

          [TM] The DIGITAL Logo, HSC, RA, RD, VAX, VAXcluster and VMS are
               trademarks of Digital Equipment Corporation.


















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