HP Volume Shadowing for OpenVMS |
Creating and Managing Shadow Sets Using DCL Commands |
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Managing Copy and Merge Operations (Alpha Only)
The SET SHADOW command, introduced in OpenVMS Alpha Version 7.3-2, provides these controls. All SET SHADOW qualifiers pertain to shadow sets, and some can also be applied to individual shadow set members, as described in SET SHADOW Command Qualifiers for Multiple-Site Shadow Set Members. The qualifiers remain in effect until the device (shadow set or shadow set member) is dismounted. If the device is remounted (in the case of a shadow set member, returned to the shadow set from which it was dismounted), the qualifier must be specified again. The SET SHADOW command requires the VOLPRO privilege.
These qualifiers act upon the shadow set or the shadow set member that you specify, as described in SET SHADOW Command Qualifiers for Multiple-Site Shadow Set Members. The following example shows how to specify qualifiers for a shadow set (DSAnnnn:):
$ SET SHADOW DSAnnnn:/qualifier/qualifier
Some of these qualifiers are also available for the SET DEVICE command, as shown in SET DEVICE Command Qualifiers for Multiple-Site Shadow Set Members, and perform the same function. If the qualifier is also available for the SET DEVICE command, it is noted here.
| Qualifier | Function |
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/ABORT_VIRTUAL_UNIT DSAnnnn:
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Aborts mount verification immediately, provided
the shadow set is in mount verification on the node where the command
is issued. If the shadow set is not in mount verification, this
command returns the error %SYSTEM-E-UNSUPPORTED, unsupported operation
or function. This qualifier is intended for use when the shadow
set cannot be recovered. After this command completes, the shadow
set must still be dismounted. To do so, use the following command: $ DISMOUNT/ABORT
DSAnnnn: This qualifier is also available for the SET DEVICE command. |
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/COPY_SOURCE {ddcu:, DSAnnnn:}
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Controls whether one (ddcu:) or both (DSAnnnn)
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 qualifier affects only copy operations that do
not use disk copy data (DCD) commands.Some storage controllers,
such as the HSG80, 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, specified source member for a copy operation.If
only a shadow set (DSAnnnn) is specified, all reads for full copy
operations will be performed from the disk that is the current "master"
member, regardless of physical location of the disk.If a shadow
set member (ddcu:) is specified, 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.This qualifier is also
available for the SET DEVICE command. |
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/DEMAND_MERGE
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Initiates a merge operation on the specified
shadow set. This qualifier is useful if the shadow set was created
with the INITIALIZE/SHADOW command without the use of the /ERASE
qualifier. For more information about this qualifier, see
Using /DEMAND_MERGE to Start a Merge Operation. |
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/FORCE_REMOVAL ddcu
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Expels the specified shadow set member
from the shadow set.
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 specified device must be a member of a shadow set that is mounted on the node where the command is issued. |
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/LOG
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Instructs the volume shadowing software
to display a brief message that confirms that the SET SHADOW command
completed. If /OUTPUT is also specified, this information is written
to the output file.
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/MEMBER_TIMEOUT =n ddcu:
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Specifies the timeout value to be used for
a shadow set member. The value supplied by this qualifier overrides
the system 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 n is 1 to 16777215 seconds.The specified
device must be a member of a shadow set that is mounted on the node
where the command is issued. This qualifier is also available for
the SET DEVICE command. |
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/MVTIMEOUT= n DSAnnnn:
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Specifies the mount verification timeout
value to be used for this shadow set, specified by its virtual unit
name, DSAnnnn:.The value supplied by this qualifier overrides the
value specified by the system parameter MVTIMEOUT for this specific
shadow set.The valid range for n is 1 to 16777215 seconds. The specified
shadow set must be mounted on the node where the command is issued.
After you apply this qualifier, the setting remains in effect as long as the shadow set is mounted. If the shadow set is dismounted and later remounted, this qualifier must be specified again.This qualifier is also available for the SET DEVICE command. |
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/OUTPUT= file-name
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Outputs any messages to the specified file. |
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/READ_COST=n ddcu:
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Enables you to modify the default "cost" assigned
to each member of a shadow set. By modifying the assignments, you
can bias the reads in favor of one member of a two-member shadow
set, or, in the case of three-member shadow sets, in favor of one
or two members of the set over the remaining members.
The device specified must be a member of a shadow set that is mounted on the node where the command is issued. The valid range for n is 1 to 65,535 units. 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. The following list of device types is ordered by the default READ_COST assignments, from the lowest cost to the highest cost:
The value supplied by the /READ_COST 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. After you have applied this qualifier to a member, the setting remains in effect as long as the member is part of the shadow set. If the member is removed from the shadow set and later returned, this qualifier must be specified again. 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 takes site values into account when it assigns default READ_COST values. In order for the shadowing software to determine whether 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 the shadow set member. 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. This qualifier is also available for the SET DEVICE command. |
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/READ_COST = n DSAnnnn:
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Switches the read cost setting for all
shadow set members back to the default read cost settings established
automatically by the shadowing software.
The valid range for n is any number. The value supplied has no inherent meaning. DSAnnnn must be a shadow set that is mounted on the node from which this command is issued. This qualifier is also available for the SET DEVICE command. |
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/SITE = n (ddcu:, DSAnnnn:)
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Sets the site value either for the shadow
set, represented by its virtual unit name, or for a specified shadow
set member. (This qualifier is also available for the SET DEVICE
command.)
The SHADOW_SITE_ID system parameter defines the default site location of the shadow set. You can override the default location of the shadow set with this qualifier. The valid range for the site location, represented by n is 1 through 255. After you apply this qualifier, the setting remains in effect until you change it either with this command or with the SET DEVICE/SITE command. This qualifier can improve read performance because the member that is physically local to the system will be the preferred disk from which to read, provided that you specify the /SITE qualifier for each shadow set member and for the shadow set. This qualifier is also available for use with the SET SHADOW command, although SET SHADOW does not support the use of logical names.) |
Using
/DEMAND_MERGE to Start a Merge Operation ![]()
The /DEMAND_MERGE qualifier was created to force a merge operation
on shadow sets that were created with the INITIALIZE/SHADOW command
without specifying the /ERASE qualifier. The /DEMAND_MERGE qualifier
ensures that all blocks not in use by active files are the same.
The system manager can enter this command at a convenient time.
If the /ERASE qualifier was not used when the shadow set was created
with /INITIALIZE/SHADOW, and the SET SHADOW/DEMAND_MERGE command
has not been executed, then the higher overhead of a full merge
operation on this shadow set will be encountered after a system
failure.
System managers can also use the SET SHADOW/DEMAND_ MERGE command if the ANALYZE/DISK/SHADOW command found differences between the members of the shadow set (see Using ANALYZE/DISK/SHADOW to Examine a Shadow Set).
SHOW SHADOW
Management Functions ![]()
The SHOW SHADOW command reports on the status of the specified
shadow set and indicates whether a merge or copy operation is required,
depending on the qualifier that you specify. If a merge or copy
operation is required, this command reports whether it is pending
or in progress. The qualifiers are described in this section.
To use this command, specify the shadow set's virtual
unit name, followed by the qualifiers you want to use, as shown
in the following example: $ SHOW SHADOW DSAnnnn:/qualifier/qualifier/
/ACTIVE ![]()
This qualifier returns one of three possible states:
/COPY ![]()
This qualifier returns one of three possible states:
/MERGE ![]()
This qualifier returns one of three possible states:
/OUTPUT=file-name ![]()
This qualifier outputs any messages to the specified file.
SHOW SHADOW Sample Output shows sample output from the SHOW SHADOW command:
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