| Document revision date: 30 March 2001 | |
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This chapter describes how to create, mount, dismount, and dissolve shadow sets using interactive DCL commands. It also describes how to use the DCL command SET DEVICE to specify management attributes for shadow set members located at different sites in a multiple-site OpenVMS Cluster system. In addition, it describes how to use the DCL command SHOW DEVICE and the lexical function F$GETDVI to access current information about the state of shadow sets.
Volume Shadowing for OpenVMS improves data availability by ensuring that corresponding logical block numbers (LBNs) on multiple disk volumes contain the same information. Upon receiving a command to mount or dismount disks in a shadow set, the volume shadowing software may need to reconcile data differences and ensure that corresponding LBNs contain the same information.
An understanding of the copy and merge operations used for data
reconciliation is essential to the discussions in this chapter.
Therefore, you may find it helpful to refer to
<REFERENCE>(MEMBERSHIP_CH) to understand how Volume Shadowing for
OpenVMS ensures data availability and consistency during changes in
shadow set membership.
1.1 Allocating Devices
To avoid the possibility of another user mounting a particular device before you enter the MOUNT command, you can optionally allocate the device before issuing the MOUNT command. Use the DCL command ALLOCATE to provide your process with exclusive access to a physical device until you either deallocate the device or terminate your process. Optionally, you can associate a logical name with the device. The format for the ALLOCATE command is as follows:
|
ALLOCATE device-name[:] logical-name[:] |
To create a shadow set, you must use the MOUNT command with the /SHADOW qualifier to mount at least one physical disk into a shadow set and assign a virtual unit name to the set, as shown in Example 1-1.
| Example 1-1 Creating a Shadow Set | ||
|---|---|---|
$ MOUNT DSA23:(1) /SHADOW(2) =$4$DUA9:(3) volume_label (4) logical_name (5) |
This example forms a shadow set represented by the virtual unit DSA23, and includes one shadow set member, $4$DUA9.
In addition, you can specify /SYSTEM, /GROUP, or /CLUSTER to make the
shadow set available to all users of a system, all members of a group,
or all nodes in a cluster on which shadowing is enabled.
To create a three-member shadow set, you can add two members in a
single MOUNT command to an existing one-member shadow set. This method
optimizes the I/O operation because both members are copied at the same
time. (See the example in Section 1.3.4.)
Upon receiving a command to create a shadow set, the volume shadowing
software may perform a copy or a merge operation to reconcile data
differences. If you are not sure which disks might be targets of copy
operations, you can specify the /CONFIRM or /NOCOPY qualifiers as a
precaution against overwriting important data when you mount a disk.
These and other MOUNT command qualifiers are discussed in Section 1.3.
1.3 MOUNT Command Qualifiers for Shadowing
This section briefly describes the MOUNT command qualifiers that are useful for shadow set management. Refer also to the OpenVMS System Management Utilities Reference Manual for complete information about these and other DCL commands.
You must use the /SHADOW qualifier when you create a new shadow set or when you add a member to an existing shadow set. You can also use the optional qualifiers described in Table 1_1 and in Table 1-2. These qualifiers require the VOLPRO and OPER privileges, or your user identification code (UIC) must match the owner UIC of the volume being mounted. To mount a shadow set throughout the system, you must also have the SYSNAM privilege.
Detailed examples and descriptions of how to use these qualifiers are
included in Section 1.4.
In addition to the shadowing-specific qualifiers described in
Table 1-1, the /NOASSIST, /SYSTEM, /GROUP, and /CLUSTER qualifiers
are also frequently used when mounting shadow sets, as described in
Table 1_2 and in Section 1.3.2.
1.3.1 MOUNT Command Qualifiers Specific to Shadowing
The MOUNT command qualifiers described in Table 1-1 are specific to shadowing.
| Qualifier | Function |
|---|---|
| /[NO]CONFIRM | Controls whether the Mount utility issues a request to confirm a copy operation when mounting a shadow set. The default is /NOCONFIRM. |
| /[NO]COPY | Enables or disables copy operations on physical devices named when mounting or adding to a shadow set. The default is /COPY. |
| /[NO]INCLUDE | Automatically mounts and reinstates a shadow set to the way it was before the shadow set was dissolved. The default is /NOINCLUDE. |
| /OVERRIDE=NO_FORCED_ERROR | Directs the Mount utility to proceed with shadowing, even though the device or controller does not support forced error handling. Using unsupported SCSI disks can cause members to be removed from a shadow set if certain error conditions arise that cannot be corrected, because some SCSI disks do not implement READL and WRITEL commands that support disk bad-block repair. If the SCSI device does not support READL and WRITEL commands, the SCSI disk class driver sets a NOFE (no forced error) bit in a System Dump Analyzer display. See Section 1.8.4.1 for more information. |
| /OVERRIDE=SHADOW_MEMBERSHIP | Mounts a former shadow set member and zeroes the disk's shadow set generation number so that the disk is no longer marked as having been a member of the shadow set. |
| /POLICY= [NO]MINICOPY [=OPTIONAL] |
Controls the setup and use of the shadowing minicopy function. The
meaning of [NO]MINICOPY[=OPTIONAL] depends on the status of the shadow
set. If the shadow set is not mounted, either on a standalone system or
on any cluster member, and MINICOPY=OPTIONAL is specified, the shadow
set is mounted and a write bitmap is created. (A write bitmap enables a
shadowing minicopy operation.) MOUNT/POLICY=MINICOPY[=OPTIONAL] must be
specified on the initial mount of a shadow set, either on a standalone
system or in a cluster, to enable the shadowing minicopy operation.
The OPTIONAL keyword allows the mount to continue, even if the system was unable to start the write bitmap. A bitmap could fail to start properly because of an improperly dismounted shadow set, a shadow set that requires a merge operation, or various resource problems. If the OPTIONAL keyword is omitted and the system is unable to start the write bitmap, the shadow set will not be mounted. If you specify /POLICY=MINICOPY=OPTIONAL and the shadow set was already mounted on another node in the cluster without this qualifier and keyword, the MOUNT command will succeed but a write bitmap will not be created. If NOMINICOPY is specified, the shadow set will be mounted but a write bitmap will not be created. If a former member of the the shadow set is returned to the shadow set, which has minicopy enabled, then a minicopy is started instead of a full copy. This is the default behavior and will occur even if you omit /POLICY=MINICOPY[=OPTIONAL]. If a minicopy successfully starts and then fails for some reason, a full copy will be performed. If a minicopy cannot be started and the keyword OPTIONAL was omitted, the mount will fail. If NOMINICOPY is specified, then a minicopy will not be performed, even if one is possible. |
| /POLICY=REQUIRE_MEMBERS | Controls whether every physical device specified with the /SHADOW qualifier must be accessible when the MOUNT command is issued in order for the MOUNT command to take effect. The proposed members are either specified in the command line or found on the disk by means of the /INCLUDE qualifier. The default behavior is that if one or more members is not accessible for any reason (such as a connectivity failure), then the virtual unit will be created with the remaining members. This option is especially useful in disaster-tolerant clusters because it ensures that the correct member is selected as the master member after an event. |
| /POLICY=VERIFY_LABELS |
Requires that any member to be added to the shadow set have a volume
label of SCRATCH_DISK.
This helps ensure that the wrong disk is not added to a shadow set by mistake. If you plan to use VERIFY_LABEL, then before using this qualifier you must either initialize the disk to be added to the set with the label SCRATCH_DISK or specify a label for the disk with the command SET VOLUME/LABEL. The default behavior is NOVERIFY_LABEL, which means that the volume label of the copy targets will not be checked. This is the same behavior that occurred before the introduction of this qualifier. The volume label of the copy targets will not be checked. |
| /SHADOW=( physical-device-name[:][,...]) | Directs the Mount utility to bind the specified physical devices into a shadow set represented by the virtual unit named in the command. |
The MOUNT command qualifiers described in this section are not specific to shadowing but can be very useful when creating shadow sets. These additional qualifiers are described in Table 1-2 and in the examples that follow.
| Qualifier | Function |
|---|---|
| /NOASSIST | Successfully mounts a shadow set if at least one of the devices included in the MOUNT command is available for mounting. If the command does not include the /NOASSIST qualifier, and a device included in the MOUNT command is not available for mounting, the MOUNT command will mount the shadow set. |
| /SYSTEM | Makes the volume available to all users on the system. Use this qualifier when you add a disk to an existing shadow set. If the /CLUSTER qualifier was used when the shadow set was created, the use of /SYSTEM will make the new member of the shadow set available to all nodes in the cluster that already have the shadow set mounted. |
| /GROUP | Makes the volume available to all users with the same group number in their UICs as the user entering the MOUNT command. You must have GRPNAM and SYSNAM user privileges to mount group and system volumes. |
| /CLUSTER | Creates the virtual unit automatically on every node in the cluster on which shadowing is enabled. Use this qualifier if the shadow set is to be accessed across the cluster. You must have the SYSNAM privilege to use this qualifier. Using /CLUSTER automatically includes the /SYSTEM qualifier, making the shadow set available to all users on the system. |
You may occasionally find it useful to specify the /NOASSIST qualifier on the MOUNT command. For example, you can use the MOUNT/NOASSIST command in startup files to avoid failure of a MOUNT command when a device you specify in the command is not available. The /NOASSIST qualifier can be used in startup files because operator intervention is impossible during startup.
The MOUNT/NOASSIST qualifier can successfully mount the shadow set as long as at least one of the devices included in the MOUNT command is available for mounting. Example 1-2 shows an example of the /NOASSIST qualifier and the resulting messages when one of the members included in the command is not available for mounting.
| Example 1-2 Using the /NOASSIST Qualifier | ||
|---|---|---|
$ MOUNT/SYS DSA65:/SHADOW=($4$DIA6,$4$DIA5) GALEXY/NOASSIST %MOUNT-I-MOUNTED, GALEXY mounted on _DSA65: %MOUNT-I-SHDWMEMSUCC, _$4$DIA6: (READY) is now a valid member of the shadowset %MOUNT-I-SHDWMEMFAIL, $4$DIA5 failed as a member of the shadow set -SYSTEM-F-VOLINV, volume is not software enabled |
Even though device $4$DIA5 is not available for mounting, the MOUNT
command continues to create the shadow set with $4$DIA6 as its only
member. If the command did not include the /NOASSIST qualifier, the
MOUNT command would not mount the shadow set.
1.3.4 Creating a Shadow Set With /SYSTEM and With /CLUSTER
When you create a shadow set virtual unit, you must specify either the /SYSTEM qualifier or the /CLUSTER qualifier, or both (see Table 1-2) to provide access for all users on a single system or on a cluster.
In Example 1-3, if the virtual unit is not currently mounted, the first command creates a shadow set with one shadow set member; the second command adds two more members to the same shadow set. An automatic copy operation causes any data on the second and third volumes to be overwritten as the shadow set members are added.
In the second MOUNT command, you need only specify the /SYSTEM when you add the $6$DIA5 and $6$DIA6 devices to the shadow set. Do not use /CLUSTER. These disks are added with the same status that the shadow set currently has, which in this case is clusterwide access.
| Example 1-3 Using the /CLUSTER Qualifier | ||
|---|---|---|
$ MOUNT DSA2: /CLUSTER /SHADOW=$6$DIA4: PEAKSISLAND DISK$PEAKSISLAND $ MOUNT DSA2: /SYSTEM/SHADOW=($6$DIA5:,$6$DIA6:) PEAKSISLAND DISK$PEAKSISLAND |
Once a shadow set is created, you can add and remove individual members
by mounting or dismounting physical disk devices. The shadowing
software allows you to add and remove shadow set members at any time,
transparently to user processes or applications running on the system.
1.4.1 Adding a Disk to an Existing Shadow Set
Example 1-4 shows how to add the disk $4$DUA3 to the DSA23 shadow set.
| Example 1-4 Adding a Disk to an Existing Shadow Set | ||
|---|---|---|
$ MOUNT/CONFIRM/SYSTEM DSA23: /SHADOW=($4$DUA9,$4$DUA3) volume-label |
The command in Example 1-4 specifies both the currently active shadow set member ($4$DUA9) and the new member ($4$DUA3). Although it is not necessary to include them when mounting additional physical devices, you can specify current shadow set members without affecting their membership state.
Note that when you add volumes to an existing shadow set mounted across
an OpenVMS Cluster system, the shadowing software automatically adds
the new members on each OpenVMS Cluster node.
1.4.2 Creating a Two-Member Shadow Set and Adding a Third Member
Example 1-5 shows two commands. The first command creates the shadow set with two members; the next adds a third member to that shadow set.
| Example 1-5 Creating a Shadow Set and Adding Third Member | ||
|---|---|---|
$ MOUNT/SYSTEM DSA4: /SHADOW = ($3$DIA7:, $3$DIA8:) FORMERSELF(1) %MOUNT-I-MOUNTED, FORMERSELF mounted on DSA4: %MOUNT-I-SHDWMEMSUCC, _$3$DIA7: (DISK300) is now a valid member of the shadow set %MOUNT-I-SHDWMEMSUCC, _$3$DIA8: (DISK301) is now a valid member of the shadow set $ MOUNT/SYSTEM DSA4: /SHADOW = $3$DIA6: FORMERSELF(2) %MOUNT-I-SHDWMEMCOPY, _$3$DIA6: (DISK302) added to the shadow set with a copy operation |
When you add a disk to an existing shadow set, a copy operation is necessary. Volume shadowing automatically performs the copy operation, unless you use the /CONFIRM qualifier or the /NOCOPY qualifier. When you specify the /CONFIRM qualifier, as shown in Example 1-6, the MOUNT command displays the targets of copy operations and requests permission before the operations are performed. This precaution prevents the erasing important data. For more information about copy operations, see <REFERENCE>(membership_ch).
| Example 1-6 Using the /CONFIRM Qualifier | ||
|---|---|---|
$ MOUNT/CONFIRM DSA23: /SHADOW=($1$DUA4:,$1$DUA6:) SHADOWVOL (1) %MOUNT-F-SHDWCOPYREQ, shadow copy required Virtual Unit - DSA23 Volume Label - SHADOWVOL (2) Member Volume Label Owner UIC (3) $1$DUA6: (LOVE) SCRATCH [100,100] Allow FULL shadow copy on the above member(s)? [N]: NO (4) $ |
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