Shadow Set

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A shadow set is a set of volumes that provides data replication through OpenVMS Volume Shadowing. Shadow sets are seen in the output of SHOW DEVICE as DSx devices.

Initializing Shadow Set Members

INITIALIZE/SHADOW=(device_name_1, device_name_2, device_name_3) label initializes multiple members of a future shadow set. While it is not required to initialize members of a shadow set in this way, this eliminates the requirement of a full copy when you later create a shadow set.

When both the /SHADOW and /ERASE qualifiers are specified, the INITIALIZE command performs the following operations:

  • Formats up to six devices with one command, so that any three can be subsequently mounted together as members of a new host-based shadow set
  • Writes a label on each volume
  • Deletes all information from the devices except for the system files and leaves each device with identical file structure information. All former contents of the disks are lost.

VSI strongly recommends that you use the /ERASE' qualifier. When /ERASE is specified, a merge operation is substantially reduced. However, using /ERASE has two side effects that are important considerations for volume shadowing: the setting of the ERASE volume attribute and the time it takes to initialize a volume using /ERASE.

If /ERASE is specified with /SHADOW, the disks are erased sequentially, which effectively doubles or triples the time it takes for the command to complete. If the disks are large, consider performing multiple, simultaneous INITIALIZE/ERASE commands (without /SHADOW) to erase the disks. Once all of those commands have completed, then execute an INITIALIZE/SHADOW command (without /ERASE).

Once you have initialized your devices using /ERASE and /SHADOW, you can then mount up to three of these devices as members of a new host-based shadow set.

Note that the INITIALIZE/SHADOW command should not be used to initialize a disk to be added to an existing shadow set, as no benefit is gained.

Building a Shadow Set

MOUNT/SHADOW binds devices into a shadow set. The syntax is as follows:

MOUNT (virtual-unit-name[:] /SHADOW=(physical-device-name[:][,...]))

This binds up to three physical devices into a shadow set represented by the virtual unit named in the command. This qualifier indicates that you are mounting a shadow set including the physical devices and the virtual unit that represents them to the system. This qualifier instructs MOUNT to expect a virtual unit name as the device-name parameter. Place the /SHADOW qualifier after the virtual-unit-name parameter. Use the virtual unit naming format DSAn, where n is a unique number from 0 to 9999. For the physical-device-name, use the standard device-naming format $allocation-class$ddcu[:].

MOUNT/SHADOW/INCLUDE automatically reconstructs a former shadow set to the way it was before the shadow set was dissolved.

Removing a Unit from a Shadow Set

You can remove one of the shadow set members from the shadow set with a DISMOUNT command. However, when you mount it outside of the shadow set, the volume will be write locked. To ignore shadow membership on mount, use MOUNT/OVERRIDE=SHADOW_MEMBERSHIP.

On the DISMOUNT command, you can specify /POLICY=MINICOPY to have a shadow bitmap created to speed up a subsequent copy operation. See Minicopy for more information on minicopy.

Returning a Unit to a Shadow Set

To return a unit to the shadow set, use the MOUNT/SHADOW command. Unless you use MOUNT/NOCOPY, a copy operation will begin; you can speed up the copy by creating a shadow bitmap upon dismount of a shadow member.

MOUNT/SHADOW/CONFIRM causes MOUNT to pause and request confirmation before performing a copy operation on the specified disk device.

Dissolving a Shadow Set

You can dissolve a shadow set with the DISMOUNT command. Note that after you dissolve the shadow set, the DSA device and the former members of the shadow set will stay online until the system is rebooted.

Displaying Shadow Set Information

Shadow sets are seen as DSx devices in the output of SHOW DEVICE command. Shadow set members are labeled "ShadowSetMember" and have "member of ..." instead of their label in the device listing.

Device                  Device           Error   Volume          Free  Trans Mnt
 Name                   Status           Count    Label         Blocks Coun
DSA0:                   Mounted              0 XXXXXXXXXXX    57282784   257   3
DSA1:                   Mounted              0 XXXXXXXXXX    152812992     5   3
DSA6:                   Mounted              0 XXXXXXX         3905728     1   3
$1$DGA1:      (NODEXX)  ShadowSetMember      1 (member of DSA0:)
$1$DGA2:      (NODEXX)  ShadowSetMember      1 (member of DSA0:)
$1$DGA10:     (NODEXX)  ShadowSetMember      0 (member of DSA1:)
$1$DGA20:     (NODEXX)  ShadowSetMember      0 (member of DSA1:)
 

SHOW DEVICE also reflects the status of copy or merge operations in progress.

SHOW SHADOW [DSAn[:]] displays the status of the specified shadow set or a summary of the device. SHOW SHADOW/OUTPUT[=filespec] outputs any messages to the specified file.

            $ SHOW SHADOW DSA112:
           _DSA112:  Volume Label: DSA112
             Virtual Unit State:   Steady State
             No Enhanced Shadowing Features in use

             VU Timeout Value      1200    VU Site Value          0
             Copy/Merge Priority   5000    Mini Merge      Disabled
             Served Path Delay     30

             Device $1$DGA112              Master Member
               Read Cost             2     Site 0
               Member Timeout        180

             Device $1$DGA113
               Read Cost             2     Site 0
               Member Timeout        180
 
           $ SHOW SHADOW DSA112:
           _DSA112:  Volume Label: DSA112
           Virtual Unit State: MiniMerge Active (5%),Copy Pending (32%) on NODE1
             Enhanced Shadowing Features in use:
                 Host-Based Minimerge (HBMM)

             VU Timeout Value      1200    VU Site Value          0
             Copy/Merge Priority   5000    Mini Merge       Enabled
             Served Path Delay     30

             HBMM Policy
               HBMM Reset Threshold: 50000
               HBMM Master lists:
                 Up to any 6 nodes in the cluster
               HBMM bitmaps are active on NODE1,NODE2
               Modified blocks since bitmap creation: 137033

             Device $1$DGA112              Master Member, Merging (5%)
               Read Cost             2     Site 0
               Member Timeout        180

             Device $1$DGA113              Copy Target (32%)
               Read Cost             2     Site 0
               Member Timeout        180
  

SHOW SHADOW/FULL displays complete information about the specified devices (or all devices if no device is specified). The only other qualifier that can be specified with /FULL is /OUTPUT.

$ SHOW SHADOW DSA12 /FULL

           _DSA12:   VolumeLabel: DGA12
             Virtual Unit State:  Steady State
             Enhanced Shadowing Features in use:
                 Host-Based Minimerge (HBMM)

             Cluster Virtual Unit Status: 0001 - normal
             Local Virtual Unit Status: 00000110 - Local Read,Policy Active

             Total Devices         2       VU_UCB          81435180
             Source Members        2       SCB LBN         003FEB5F
             Act Copy Target       0       Generation      00A2BA31
             Act Merge Target      0        Number         441DA800
             Last Read Index       0       Master Mbr Index       0
             Copy Hotblocks        0       Copy Collisions        0
             SCP Merge Repair Cnt  0       APP Merge Repair Cnt   0
             VU Timeout Value      300     VU Site Value          0
             Copy/Merge Priority   5000    Mini Merge       Enabled
             Served Path Delay     0

             HBMM Policy
               HBMM Reset Threshold: 50000
               HBMM Master lists:
                 Up to any 2 of the nodes: XYZ100,XYZ200
               HBMM bitmaps are active on the following nodes: XYZ200,XYZ100
             Modified blocks since bitmap creation: 254

             Device $1$DGA12               Master Member
               Index 0 Status  000000A0    src,valid
               Ext. Member Status    00
               Read Cost             2     Site 0
               Member Timeout        180   UCB  81643840

             Device $1$DGA22
               Index 1 Status  000000A0    src,valid
               Ext. Member Status    00
               Read Cost             2     Site 0
               Member Timeout        180   UCB  81642840

             Device $1$DGA32
               Index 1 Status 000000A0     src,valid
               Ext. Member Status    00
               Read Cost             2     Site  0
               Member Timeout        180   UCB  81641840

 

Active Operations

SHOW SHADOW /ACTIVE displays one of three possible states:

  • Merge or Copy is not required
  • Copy is in progress on node node-name, n% completed
  • Merge is in progress on node node-name, n% completed
           $ SHOW SHADOW DSA1010 /ACTIVE

           Device     Volume Name   Status

           _DSA1010:  FOOBAR        Merge Active (25%) on NODE1
 

SHOW SHADOW/BY_PRIORITY [DSAn:] displays a list of shadow sets on the system, sorted in the order in which a recovery operation (copy, full merge, or minimerge) is scheduled to occur. If a copy or merge is active, the display shows on which system it is occurring. This list is specific to the system on which the command is executed, so the list might vary across systems in a cluster. Higher numbers indicate a higher priority.

           $ SHOW SHADOW /BY_PRIORITY
           Device    Mbr                                        Active
            Name     Cnt  Priority Virtual Unit State          on Node
           _DSA30:    2     5000   Steady State
           _DSA12:    3     4000   Steady State
           _DSA234:         3000   Not Mounted on this node
           Total of 2 Operational shadow sets; 0 in Mount Verification;
           1 not mounted
 

Tip: When the command specifies a virtual unit (DSAn:), the display shows the current priority and status of just this virtual unit. If any copy or merge operations are in progress, the system on which the operation is occurring is also displayed, along with the operation's progress.

           $ SHOW SHADOW /BY_PRIORITY DSA11
           Device    Mbr                                           Active
            Name     Cnt  Priority  Virtual Unit State            on Node
           _DSA11:    3     5000    Merge Active (29%)              NODE3

 


SHOW SHADOW/COPY displays one of three possible states:

  • Copy is not required
  • Copy is pending
  • Copy is in progress on node node-name, n% completed

SHOW SHADOW/MERGE displays one of three possible states:

  • Merge is not required
  • Merge is pending
  • Merge is in progress on node node-name, n% completed
           $ SHOW SHADOW /MERGE

           Device    Volume Name Status
           _DSA1010: FOOBAR      Merge Pending (23%), Copy Active (77%) on NODE1

 

SHOW SHADOW/SITE=n displays every shadow set member device (and the virtual device of which it is a member) that has the site value specified by n.

Minimerge

SHOW SHADOW/POLICY=HBMM displays the host-based minimerge (HBMM) policy associated with the specified devices. HBMM is the only value supported for /POLICY, and it must be included.

            $ SHOW SHADOW DSA12 /POLICY=HBMM

             HBMM Policy for device _DSA12:
               HBMM Reset Threshold: 50000
               HBMM Master lists:
                 Up to any 2 of the nodes: NODE1,NODE2,NODE3
               HBMM bitmaps are active on the following nodes: NODE2,NODE1
             HBMM Reset Count      4      Last Reset  9-JAN-2004 13:40:11.45
             Modified blocks since last bitmap reset: 142367
 

If the command is issued with no device specification, information is output for all devices. There might be a noticeable delay between the output of device information or before returning to the DCL command prompt. This delay is normal.

$ SHOW SHADOW /POLICY=HBMM

              HBMM Policy for device _DSA12:
                HBMM Reset Threshold: 50000
                HBMM Master lists:
                  Up to any 2 of the nodes: NODE1,NODE2
                HBMM bitmaps are active on NODE1,NODE2
              Modified blocks since bitmap creation: 254

              HBMM Policy for device _DSA30:
                HBMM Reset Threshold: 50000
                HBMM Master lists:
                  Up to any 2 of the nodes: FLURRY,FREEZE,HOTTUB

              HBMM Policy for device _DSA99:
                HBMM Reset Threshold: 50000
                HBMM Master lists:
                  Up to any 2 of the nodes: NODE1,NODE2,NODE3
                  Any 1 of the nodes: NODE4,NODE5
                  Up to any 2 of the nodes: ICEBAR,SWEDEN,MILANO

              HBMM Policy for device _DSA999:
                HBMM Reset Threshold: 50000
                HBMM Master lists:
                  Up to any 2 of the nodes: NODE1,NODE2,NODE3
                  Any 1 of the nodes: NODE4,NODE5
                  Up to any 2 of the nodes: ICEBAR,SWEDEN,MILANO
 

Use /NAME[=policy-name] with /POLICY=HBMM to display the characteristics of a specific, named host-based minimerge (HBMM) policy or, if no policy name is specified, all named HBMM policies on the cluster.

              $ SHOW SHADOW/POLICY=HBMM/NAME=PEAKS_ISLAND

              HBMM Policy PEAKS_ISLAND
                HBMM Reset Threshold: 50000
                HBMM Master lists:
                  Up to any 2 of the nodes: NODE1,NODE2,NODE3
                  Any 1 of the nodes: NODE4,NODE5
                  Up to any 2 of the nodes: NODE6,NODE7,NODE8
 

See also