Document revision date: 30 March 2001
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1.4.4 Checking Status of Potential Shadow Set Members With /NOCOPY

When you specify more than one disk, the shadowing software automatically determines the correct copy operation to perform in order to make shadow set members consistent with each other (see <REFERENCE>(copy_op_sect) for details). The Mount utility interprets information recorded on each member to determine whether a member requires a copy operation, a merge operation, or no copy operation. If you are not sure which disks might be targets of copy operations, you can specify the /CONFIRM qualifier or the /NOCOPY qualifier as a precaution against overwriting important data when you mount a disk. With the /NOCOPY qualifier, you disable the copy operation.

Example 1-7 shows how to use the /NOCOPY qualifier to check the status of potential shadow set members before any data is erased.

Example 1-7 Using the /NOCOPY Qualifier

$ MOUNT/NOCOPY DSA2: /SHADOW=($1$DUA4:,$1$DUA6:,$1$DUA7:) -
_$ SHADOWVOL DISK$SHADOWVOL (1)
%MOUNT-F-SHDWCOPYREQ, shadow copy required
%MOUNT-I-SHDWMEMFAIL, DUA7: failed as a member of the shadow set
%MOUNT-F-SHDWCOPYREQ, shadow copy required (2)
$  MOUNT/COPY(3) DSA2: /SHADOW=($1$DUA4:,$1$DUA6:,$1$DUA7:) -
_$ SHADOWVOL DISK$SHADOWVOL 
%MOUNT-I-MOUNTED, SHADOWVOL    mounted on _DSA2: 
%MOUNT-I-SHDWMEMSUCC, _$1$DUA4: (VOLUME001) is now a valid member of
                      the shadow set
%MOUNT-I-SHDWMEMSUCC, _$1$DUA6: (VOLUME002) is now a valid member of
                      the shadow set
%MOUNT-I-SHDWMEMCOPY, _$1$DUA7: (VOLUME003) added to the shadow set
                      with a copy operation (4)
 

  1. This command instructs MOUNT to build a shadow set, with the specified devices, but only if a copy or merge operation is not required.
  2. MOUNT did not build the shadow set because the specified disk, loaded on device $1$DUA7, required a copy operation. At this point you can verify that the volume in device $1$DUA7 does not contain any useful data.
  3. If the device does not contain valuable data, you can reenter the MOUNT command and include the /COPY qualifier. This command instructs MOUNT to mount a shadow set and to proceed with the necessary copy or merge operation.
  4. The shadow set is successfully mounted. The $1$DUA7 device is currently the target of a copy operation; it will attain full shadow set membership when the copy operation completes.

1.5 Mounting a Shadow Set on Other Nodes in the Cluster

If a shadow set is already mounted on one or more nodes in an OpenVMS Cluster system, the /SHADOW qualifier is not required when you mount the same shadow set on other nodes in the cluster. For example, if DSA42 is already mounted in the cluster when a new node is brought into the cluster, you can use the following command to mount DSA42 on the new node:


$ MOUNT/SYS DSA42:  volume-label logical-name

Upon receiving this command, the volume shadowing software creates the virtual unit on the new node with the same members that currently exist in the cluster.

1.5.1 Reconstructing a Shadow Set With /INCLUDE

Example 1-8 shows how to reconstruct a shadow set. The volume shadowing software determines which disk volumes are former members of the shadow set.

Example 1-8 Reconstructing Shadow Sets With /INCLUDE

$ MOUNT /SYSTEM DSA4/SHAD=($4$DIA1,$4$DIA2,$4$DIA3) NEWDISK(1)
%MOUNT-I-MOUNTED, NEWDISK   mounted on _DSA4:
%MOUNT-I-SHDWMEMSUCC, _$4$DIA1: (DISK01) is now a valid member 
                          of the shadow set
%MOUNT-I-SHDWMEMCOPY, _$4$DIA2: (DISK02) added to the shadow set 
                          with a copy operation
%MOUNT-I-SHDWMEMCOPY, _$4$DIA3: (DISK03) added to the shadow set 
                          with a copy operation
$ DISMOUNT DSA4(2)
$ 
$ MOUNT DSA4:/SYSTEM/SHAD=$4$DIA1 NEWDISK/INCLUDE(3)
%MOUNT-I-MOUNTED, NEWDISK   mounted on _DSA4:
%MOUNT-I-SHDWMEMSUCC, _$4$DIA1: (DISK01) is now a valid member (4)
                          of the shadow set
%MOUNT-I-AUTOMEMCOPY, _$4$DIA2: (DISK02) automatically added (4)
                          to the shadow set
%MOUNT-I-AUTOMEMCOPY, _$4$DIA3: (DISK03) automatically added (4)
                          to the shadow set
 

  1. This is the original MOUNT command that created the shadow set represented by DSA4. The shadow set consists of three shadow set members: $4$DIA1, $4$DIA2, and $4$DIA3.
  2. After all copy operations have completed, the DISMOUNT command dissolves the shadow set.
  3. The /INCLUDE qualifier triggers the MOUNT command to reconstruct the shadow set back to the way it was before the shadow set was dissolved. The MOUNT command must specify the original virtual unit name (DSA4) and at least one of the original shadow set members ($4$DIA1). The Mount utility reads the membership list on $4$DIA1 (specified in the MOUNT command) to determine that $4$DIA2 and $4$DIA3 are also members of the shadow set.
  4. Because the shadow set was properly dismounted, the shadow set members are in a consistent state. The MOUNT status messages indicate that the shadow set devices are added back into the shadow set without the need for copy operations.

1.5.2 Mounting a Former Shadow Set Member as a Nonshadowed Disk

Occasionally, you will need to mount a physical shadow set member as a nonshadowed disk. By default, when a shadow set member is mounted outside a shadow set, the Mount utility automatically write-locks the disk. This provides a safeguard against accidental modification, thereby allowing the disk to be remounted into a shadow set at a later time.

To override this default behavior, include the /OVERRIDE=SHADOW_MEMBERSHIP qualifier on the MOUNT command as shown in the following example:


$ MOUNT/OVERRIDE=SHADOW_MEMBERSHIP $4$DUA20: WORKDISK

This command ignores shadow set membership status and mounts a former shadow set member on $4$DUA20 as a nonshadowed disk with write access.

1.6 Specifying Disaster-Tolerant Management Attributes (Alpha Only)

Starting with OpenVMS Alpha Version 7.3, qualifiers to the DCL command SET DEVICE are provided for specifying management attributes for shadow set members located at different sites. By using these qualifiers, system managers can override the default Volume Shadowing actions that can occur when the systems at one site of a disaster-tolerant OpenVMS Cluster configuration fail. These qualifiers are designed primarily for use in a configuration that uses Fibre Channel for a site-to-site storage interconnect. They can be used in other configurations as well.

Similarly, the DCL command DISMOUNT has been enhanced by the addition of the qualifier, /FORCE_REMOVAL ddcu:. This qualifier has been added for the same purpose---to give system managers greater control of shadow set members located at different sites. For more information about this qualifier, see Section 1.7.1.

Table 1-3 SET DEVICE Command Qualifiers for Multiple-Site Shadow Set Members
Qualifier Function
/FORCE_REMOVAL ddcu: If connectivity to a device has been lost and the shadow set is in mount verification, this qualifier causes the member to be expelled from the shadow set immediately.

If the shadow set is not currently in mount verification, no immediate action is taken. If connectivity to a device has been lost but the shadow set is not in mount verification, this qualifier lets you flag the member to be expelled from the shadow set, as soon as it does enter mount verification. The device specified must be a member of a shadow set that is mounted on the node where the command is issued.

/MEMBER_TIMEOUT = xxxxxx ddcu: Specifies the timeout value to be used for a member of a shadow set.

The value supplied by this qualifier overrides the SYSGEN parameter SHADOW_MBR_TMO for this specific device. Each member of a shadow set can be assigned a different MEMBER_TIMEOUT value.

The valid range for xxxxxx is 1 to 16,777,215 seconds.

The device specified must be a member of a shadow set that is mounted on the node where the command is issued.

/MVTIMEOUT = yyyyyy DSA nnnn: Specifies the mount verification timeout value to be used for this shadow set, specified by its virtual unit name, DSA nnnn.

The value supplied by this qualifier overrides the SYSGEN parameter MVTIMEOUT for this specific shadow set.

The valid range for yyyyyy is 1 to 16,777,215 seconds. The device specified must be a shadow set that is mounted on the node where the command is issued.

/READ_COST = zzz ddcu: The valid range for zzz is 1 to 4,294,967,295 units.

The device specified must be a member of a shadow set that is mounted on the node where the command is issued.

This qualifier allows you to modify the default "cost" assigned to each member of a shadow set, so that reads are biased or prioritized toward one member versus another.

The shadowing driver assigns default READ_COST values to shadow set members when each member is initially mounted. The default value depends on the device type, and its configuration relative to the system mounting it. There are default values for a DECRAM device; a directly connected device in the same physical location; a directly connected device in a remote location; a DECram served device; and a default value for other served devices.

The value supplied by this qualifier overrides the default assignment. The shadowing driver adds the value of the current queue depth of the shadow set member to the READ_COST value and then reads from the member with the lowest value.

Different systems in the cluster can assign different costs to each shadow set member.

If the /SITE command qualifier has been specified, the shadowing driver will take site values into account when it assigns default READ_COST values. Note that in order for the shadowing software to determine if a device is in the category of "directly connected device in a remote location," the /SITE command qualifier must have been applied to both the shadow set and to the individual device.

Reads requested for a shadow set from a system at Site 1 are performed from a shadow set member that is also at Site 1. Reads requested for the same shadow set from Site 2 can read from the member located at Site 2.

/READ_COST = y DSA nnnn The valid range for y is any non-zero number. The value supplied has no meaning in itself. The purpose of this qualifier is to switch the read cost setting for all shadow set members back to the default read cost settings established automatically by the shadowing software. DSA nnnn must be a shadow set that is mounted on the node from which this command is issued.
/SITE = ( nnn, logical_name) ( ddcu: DSA nnnn:) This qualifier indicates to the shadowing driver the site location of the shadow set member or of the shadow set (represented by its virtual unit name). Prior to using this qualifier, you can define the site location in the SYLOGICALS.COM command procedure to simplify its use.

The valid range for nnn is 1 through 255.

The following example shows the site locations defined, followed by the use of the /SITE qualifier:

$ DEFINE/SYSTEM/EXEC ZKO 1

$ DEFINE/SYSTEM/EXEC LKG 2
$!
$! At the ZKO site ...
$ MOUNT/SYSTEM DSA0/SHAD=($1$DGA0:,$1$DGA1:) TEST
$ SET DEVICE/SITE=ZKO DSA0:
$!
$! At the LKG site ...
$ MOUNT/SYSTEM DSA0/SHAD=($1$DGA0,$1$DGA1) TEST
$ SET DEVICE/SITE=LKG DSA0:
$!
$! At both sites, the following would be used:
$ SET DEVICE/SITE=ZKO $1$DGA0:
$ SET DEVICE/SITE=LKG $1$DGA1:
/COPY_SOURCE ( ddcu:, DSA nnnn:) Controls whether one or both source members of a shadow set are used as the source for read data during full copy operations, when a third member is added to the shadow set. This only affects copy operations that do not use DCD operations.

HSG80 controllers have a read-ahead cache, which significantly improves single-disk read performance. Copy operations normally alternate reads between the two source members, which effectively nullifies the benefits of the read-ahead cache. This qualifier lets you force all reads from a single source member for a copy operation.

If the shadow set is specified, then all reads for full copy operations will be performed from whichever disk is the current "master" member, regardless of physical location of the disk.

If a member of the shadow set is specified, then that member will be used as the source of all copy operations. This allows you to choose a local source member, rather than a remote master member.

/ABORT_VIRTUAL_UNIT DSA nnnn: To use this qualifier, the shadow set must be in mount verification. When you specify this qualifier, the shadow set aborts mount verification immediately on the node from which the qualifier is issued. This qualifier is intended to be used when it is known that the unit cannot be recovered. Note that after this command completes, the shadow set must still be dismounted. Use the following command to dismount the shadow set:
DISMOUNT/ABORT DSA
nnnn

1.7 Removing Members and Dissolving Shadow Sets

You can remove shadow set members and dissolve shadow sets with the DCL command DISMOUNT. You must have GRPNAM and SYSNAM user privileges to dismount group and system volumes. The DISMOUNT command has the following format:

DISMOUNT {device-name[:] virtual-unit-name}

The action taken differs depending on whether you specify an individual shadow set member or the shadow set (by its virtual unit name) on the DISMOUNT command:

To dismount a shadow set that is mounted across an OpenVMS Cluster system, include the /CLUSTER qualifier with the DISMOUNT command. If you dismount a shadow set without including the /CLUSTER qualifier, only the node from which you issued the command dismounts the shadow set. The shadow set remains operational on the other OpenVMS Cluster nodes that have the shadow set mounted.

If the disks on your system are neither SCSI nor Fibre Channel disks, you can use the /NOUNLOAD qualifier on the DISMOUNT command to prevent the disk volume or volumes from spinning down. The devices remain in a ready state. If you specify the /UNLOAD qualifier when dismounting a virtual unit, the disk volumes are physically spun down after the shadow set is dissolved. See the OpenVMS DCL Dictionary for more information about using the DISMOUNT command and its qualifiers.

1.7.1 Removing Members from Shadow Sets

To remove an individual member from a shadow set, specify the name of the physical device with the DISMOUNT command. For example:


$ DISMOUNT $5$DUA7:

When you dismount an individual shadow set member, all outstanding I/O operations are completed and the member is removed from the set.

Starting with OpenVMS Alpha Version 7.3, the /FORCE_REMOVAL ddcu: qualifier is available. If connectivity to a device has been lost and the shadow set is in mount verification, /FORCE_REMOVAL ddcu: can be used to immediately expel a named shadow set member (ddcu:) from the shadow set. If you omit this qualifier, the device is not dismounted until mount verification completes. Note that this qualifier cannot be used in conjunction with the /POLICY=MINICOPY (=OPTIONAL) qualifier.

The device specified must be a member of a shadow set that is mounted on the node where the command is issued.

This qualifier gives system managers greater control of shadow sets whose members are located at different sites in an OpenVMS Cluster configuration. SET DEVICE command qualifiers are also available for specifying disaster-tolerant management attributes for shadow set members, as described in Section 1.6.

Note

You cannot dismount a device if it is the only source member in a shadow set. All shadow sets must have at least one valid source member. If you try to dismount the only source member device, the DISMOUNT command fails and returns the message:


%DISM-F-SRCMEM, Only source member of shadow set cannot be dismounted 

The only way to dismount the last source member of a shadow set is to dissolve the shadow set by specifying the virtual unit name on the DISMOUNT command.

1.7.2 Dissolving Shadow Sets

You dissolve a shadow set by specifying its virtual unit name with the DISMOUNT command. Specifying the virtual unit name causes all members in the shadow set to be dismounted, thereby dissolving the shadow set on the node that issues the DISMOUNT command. The command in Example 1-9 includes the /CLUSTER qualifier to dissolve the DSA36 shadow set across an OpenVMS Cluster system.

Example 1-9 Dissolving a Shadow Set

$ DISMOUNT /CLUSTER DSA36:

Dismounting the virtual unit can be done only after all files are closed, thereby ensuring that the dismounted disks are fully consistent from a file system perspective. The dismount operation marks the shadow set members as being properly dismounted so that a rebuild is not required the next time the disks are mounted. However, if a merge operation was either pending or in progress, then the dismount operation marks the shadow set members as being improperly dismounted and requires a merge operation.

Note

If you dismount a virtual unit while a copy operation is in progress for the shadow set, the copy operation aborts and the shadow set is dissolved. You receive OPCOM messages similar to those in the following example:


$ DISMOUNT DSA9999:
%%%%%%%%%%%  OPCOM  24-MAR-1990 20:29:57.52  %%%%%%%%%%%
$7$DUA6:  (WRKDSK) has been removed from shadow set.
%%%%%%%%%%%  OPCOM  24-MAR-1990 20:29:57.68  %%%%%%%%%%%
$7$DUA56: (PLADSK)  has been removed from shadow set.
%%%%%%%%%%%  OPCOM  24-MAR-1990 20:29:57.88  %%%%%%%%%%%
Message from user SYSTEM on SYSTMX

1.7.3 Dismounting and Remounting With One Less Member for Backup

As discussed in Section 1.7.2, the virtual unit can be dismounted on the system or across an OpenVMS Cluster system. To ensure that the virtual unit has been dismounted correctly, the following steps are recommended:

  1. Issue the MOUNT/NOWRITE command, followed by the SHOW DEVICE command, for example:


    $ MOUNT/NOWRITE DSA42: /SHADOW=($4$DUA3,$4$DUA4,$4$DUA5) volume-label 
    $ SHOW DEVICE DSA42: 
    

  2. Observe that the virtual unit is in a steady state; that is, all members are consistent and no copy or merge operation is in progress. If a copy or merge operation is in progress, you must wait for the operation to complete.
  3. When the virtual unit is in a steady state, remove a member from the shadow set with the DISMOUNT command, as shown in the following example:


    $ DISMOUNT $4$DUA5 
    

  4. Dismount the virtual unit and then remount it with one less member, as shown by the following command:


    $ MOUNT/SYS DSA42: /SHADOW=($4$DUA3,$4$DUA4) volume-label 
    

    The shadow set member that was removed can now be used for a backup operation of the virtual unit.

Note

If your application must run continuously (that is, you cannot dismount the virtual unit without disrupting your business), you can still remove a shadow set member that you plan to return later to the shadow set. Your application and recovery procedures must be designed to ensure data consistency, as described in <REFERENCE>(guidelines_shad_mem_rem_h).

1.8 Displaying Information About Shadow Sets

You can use the DCL command SHOW DEVICE or the F$GETDVI lexical function to get information about a shadow set virtual unit and the physical volumes that make up the members. You can also use the System Dump Analyzer (SDA) to get more information about shadow sets.

The following sections describe how to use these tools to examine volume shadowing virtual units and shadow set members. See also the OpenVMS DCL Dictionary for a full description of how to use the SHOW DEVICE command and the F$GETDVI lexical function. See the OpenVMS Alpha System Analysis Tools Manual and the OpenVMS VAX System Dump Analyzer Utility Manual for more information about how to use SDA on OpenVMS Alpha and OpenVMS VAX systems, respectively.

You can use any of the SHOW DEVICE qualifiers when you examine shadow sets (by specifying a shadow set's virtual unit name) or shadow set members.

Note

Because shadow sets are created and maintained individually on each node in the OpenVMS Cluster, the SHOW DEVICE display does not list shadow sets that have been created on only remote nodes.


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