Shadow Set
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.
Contents
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
