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Managing Copy and Merge Operations (Alpha Only)  



Copies and merges performed by the volume shadowing software are regulated automatically by the locking software and by the setting of SHADOW_MAX_COPIES. Many customers want greater control over the order of copies and merges; they also want to be able to specify on which nodes copy operations will take place.

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.

Table 4   SET SHADOW Command Qualifiers for Multiple-Site Shadow Set Members
Qualifier Function
/ABORT_VIRTUAL_UNIT DSAnnnn:
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.
/COPY_SOURCE {ddcu:, DSAnnnn:}
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.
/DEMAND_MERGE
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.
/FORCE_REMOVAL ddcu
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.
/LOG
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.
/MEMBER_TIMEOUT =n ddcu:
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.
/MVTIMEOUT= n DSAnnnn:
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.
/OUTPUT= file-name
Outputs any messages to the specified file.
/READ_COST=n ddcu:
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:
  • DECram device


  • Directly connected device in the same physical location


  • Directly connected device in a remote location


  • DECram served device


  • Default value for other served devices


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.
/READ_COST = n DSAnnnn:
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.
/SITE = n (ddcu:, DSAnnnn:)
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:
Example 10  SHOW SHADOW Sample Output  
$ SHOW SHADOW DSA716: 
 
_DSA716: TST716
  Virtual Unit SCB Status: 0001 - normal
  Local Virtual Unit Status: 00000010 - Local Read
 
  Total Devices         2       VU_UCB          810419C0
  Source Members        2       SCB LBN         000009C8
  Act Copy Target       0       Generation      00A15F90
  Act Merge Target      0       Number          EDA9D786
  Last Read Index       0       VU Site Value          5
  Master Mbr Index      0       VU Timeout Value    3600
  Copy Hotblocks        0       Copy Collisions        0
  SCP Merge Repair Cnt  0       APP Merge Repair Cnt   0
 
 
  Device $252$DUA716            Master Member
  Index 0 Status  000000A0      src,valid
  Ext. Member Status    00
  Read Cost             42      Site 5
  Member Timeout        120     UCB  8116FF80
 
  Device $252$DUA1010
  Index 1 Status  000000A0      src,valid
  Ext. Member Status    00
  Read Cost             500     Site 3
  Member Timeout        120     UCB  811DD500


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