Document revision date: 19 July 1999

If you do not create a volume set explicitly, the operating system creates one when necessary. If you have not mounted a volume set and a continuation volume is required, the tape file system requests that a continuation volume be mounted and implicitly creates a volume set. For example, if the tape file system encounters an EOT mark while writing a volume, it sends a message to the operator console requesting that another volume be mounted.

After you mount the next volume, the tape file system writes the volume and header labels and then reissues the pending write requests to the continuation volume. The file-set identifier in the first file-header label of all files written to the continuation volume is the file-set identifier of the first file on the first volume. The file-set identifier for volume sets is always that of the first file of the first volume that is mounted in the set.

How to Perform This Task

To explicitly create a volume set with three volumes, for example, follow these steps:

1. Allocate devices on which you will load the volumes.
2. Initialize the volumes. Specify the density and the access protection in addition to the device name and the volume identifier in the INITIALIZE commands.
3. Mount the volumes, including the device names and volume identifiers. Specifying a logical name for the volume set is optional. The system not only confirms which volumes have been mounted, but also indicates on which drive each volume has been mounted.
   The system mounts and verifies only the volumes that are physically loaded on the devices at mount time. However, the volume identifiers of additional volumes that you specify are not verified until the volumes are accessed.
4. You can check the densities, volume labels, UICs, and relative volume numbers of the volumes that are mounted on devices. To do so, specify the SHOW DEVICES/FULL command. If you specify a generic device code for the tape drives, such as MU, information is displayed for all drives of that type configured in the system.
   To display information for a volume mounted on a specific drive, specify the physical device code, consisting of the generic device code, the controller designation, and the unit number followed by a colon.
   For more information about the SHOW DEVICES command, including examples of displays returned by the SHOW DEVICES/FULL command, see Section 7.2 or the OpenVMS DCL Dictionary.

Examples

1. $ ALLOCATE MUA0: %DCL-I-ALLOC, _MARS$MUA0: allocated $ ALLOCATE MUA1: %DCL-I-ALLOC, _MARS$MUA1: allocated $ ALLOCATE MUA2: %DCL-I-ALLOC, _MARS$MUA2: allocated

   
   The commands in this example allocate a drive on which you will load each volume.

2. $ INITIALIZE/DENSITY=1600/PROTECTION=(G:RW) MUA0: TAPE1 $ INITIALIZE/DENSITY=1600/PROTECTION=(G:RW) MUA1: TAPE2 $ INITIALIZE/DENSITY=1600/PROTECTION=(G:RW) MUA2: TAPE3

   
   The commands in this example initialize the volumes. The commands specify the density and the access protection in addition to the device name and the volume identifier.

3. $ MOUNT MUA0:,MUA1:,MUA2: TAPE1,TAPE2,TAPE3 TEST %MOUNT-I-MOUNTED, TAPE1 mounted on _MUA0: %MOUNT-I-MOUNTED, TAPE2 mounted on _MUA1: %MOUNT-I-MOUNTED, TAPE3 mounted on _MUA2:

   
   The commands in this example mount the volumes. The commands include the device name and volume identifier.

When mounting a tape volume set, follow the general procedures described in Section 8.8.1. Once you create the volume set, you do not need to initialize the volumes when you mount the volume set.

Allocating a drive for each volume in the volume set is not necessary. The tape file system requests that volumes be switched to appropriate drives when continuation volumes are required.

The operating system stores, but cannot verify, the identifiers of volumes you specify but do not physically mount on drives at mount time. The system later verifies the volume identifiers when the volumes are accessed.

The operating system supports the continuous processing of mounted volumes in a tape volume set through automatic volume switching and automatic volume labeling (AVL).

8.8.2.1 Creating Labels

Depending on the following conditions, the file system does or does not create a label:

• If the file system is writing to the volume set, it creates a label for the magnetic tape and initializes the tape with that label and the protection characteristics set for the first volume of the volume set.
• If the tape file system is reading the volume set, it tries to mount the next tape in the volume set with that label.
• If the drive has no tape loaded on it, or the wrong tape, the tape file system sends a message to the operator console notifying the operator either to mount a tape or to mount the correct tape.

Before processing continuation volumes, the tape file system processes the protection on that volume (as described in Section 8.4.2). If the file system determines that the user does not have access to the volume, it sends a message to the operator.

The label fills the six-character volume identifier field:

• Characters 1 to 4 of the field contain the first four characters of the label specified for the previous volume in the volume set. (If the label is less than four characters, the volume identifier field is padded with underscores; for example, if the volume identifier is XXX, the padded field is XXX_.)
• Characters 5 and 6 contain the relative volume number for that reel in the volume set.

Note that the system can generate only 99 unique labels for a given volume set.

With automatic volume switching enabled, the operator can load a tape on the next drive allocated to the tape volume set anytime before the volume being processed reaches the EOT mark. The tape file system mounts and initializes (if INITIALIZE was specified originally) the next tape in the volume set and then notifies the operator that the switch has occurred.

8.8.2.2 Enabling Automatic Volume Switching

To use automatic volume switching, you must allocate more than one tape drive to your volume set. After you do so, the tape file system switches volumes for you automatically by selecting the next tape drive allocated to the volume set. The tape file system expects you to load the next volume in the volume set on that drive.

Examples

1. $ MOUNT MUA0:,MUA1:,MUA2: TAPE

   
   In this example, the volume with the identifier TAPE is mounted on the MUA0: drive. Load continuation volumes for this set on the tape drives in the following order: MUA1:, MUA2:, MUA0:, MUA1:, MUA2:, and so forth.

2. $ INITIALIZE MUA0: MAIN $ MOUNT/OVERRIDE=IDENTIFICATION/INITIALIZE=CONTINUATION MUA0:,MUA1:

   
   This example shows the use of the /INITIALIZE=CONTINUATION qualifier for mounting volume sets. It also shows how the system creates volume identifiers for continuation volumes.
   The volume labeled MAIN is mounted on the MUA0: drive. The second volume in the set receives the volume identifier MAIN02 and is mounted on the MUA1: drive. The third volume in the set receives the volume identifier MAIN03 and is mounted on the MUA0: drive.
   To ensure that any volume added to the tape volume set is initialized prior to being written to, mount the volume with the /INITIALIZE=CONTINUATION qualifier. The default is /NOINITIALIZE.

3. $ MOUNT MUA0:,MUA1: SUN

   
   In this example, the first volume in the set is labeled SUN and is mounted on the MUA0: drive. The second volume receives the identifier SUN_02 and is mounted on the MUA1: drive. The third volume receives the identifier SUN_03 and is mounted on the MUA0: drive.

4. $ MOUNT MUA0:,MUA1: SUN,MOON

   
   In this example, a continuation volume with two volume identifiers, SUN and MOON, is mounted on MUA0: and MUA1:, respectively. If a third volume is added to the set, it is given the identifier MOON03 and is mounted on the MUA0: drive.

If your site prelabels volumes, you must disable automatic volume switching to avoid overwriting these labels. To explicitly override automatic volume switching, specify the /NOAUTOMATIC qualifier when mounting a tape volume. (The default is /AUTOMATIC.) Note that if you allocate only one drive to the tape volume set, automatic volume switching is implicitly disabled.

When a user is reading or writing to a magnetic tape and the tape reaches end-of-tape position, the system suspends processing and sends a request to mount the next tape in the volume set. For example:

%%%%%%%%%%% OPCOM, 28-MAY-1998 15:23:31.78 %%%%%%%%%%% request 3, from user PLAW MOUNT new relative volume 2 (DW0QT2) on MUA1:

The user does not see this message and might not realize that another tape is needed to complete the read or write operation.

Example

$ MOUNT/NOAUTOMATIC MUA0: ABCD,EFGH

The command in this example tells MOUNT not to supply its own label for the second volume but, instead, to use the ones specified in the MOUNT command.

8.8.2.4 Sending Messages Back to Users

After loading the continuation volume on the drive specified in the mount request, mount the volume by entering the REPLY command with one of the three qualifiers shown in Table 8-16. For more information about these qualifiers, refer to the OpenVMS DCL Dictionary.

Table 8-16 REPLY Command Qualifiers for Continuation Volumes

Qualifier	Description
/BLANK_TAPE= identification-number	Use with an unformatted volume for write operations. This qualifier initializes the volume and requires the VOLPRO and OPER privileges to avoid a runaway tape or timeout condition. Either of the following REPLY commands is valid: $ REPLY/BLANK_TAPE=3 $ REPLY/BLANK_TAPE=3 "DW0QT2" The first command does not specify a volume identifier; the second does.
/INITIALIZE_TAPE= identification-number	Use with a formatted volume for write operations if the volume identifier on the continuation volume does not match the one specified in the mount request. The file system reinitializes the tape and mounts the volume with the new volume identifier. The tape file system then performs access checks and initializes the volume as if the INITIALIZE command had been specified. Any data on the tape prior to specifying the /INITIALIZE_TAPE qualifier is lost. The current terminal must be enabled as an operator terminal for TAPES. Either of the following commands is valid: $ REPLY/INITIALIZE_TAPE=3 $ REPLY/INITIALIZE_TAPE=3 "DW0QT2" The first command does not specify a volume identifier; the second does.
/TO= identification-number	Use with a formatted volume for both read and write operations. During a write operation, use the /TO qualifier if you want the volume identifier that is specified in the mount request to be written on the continuation volume. For example, to respond to the mount request 3, mount volume DW0QT2 on drive MTA1: and enter one of the following commands: $ REPLY/TO=3 $ REPLY/TO=3 "DW0QT2" The first command does not specify a volume identifier; the second does.

Specifying the Volume Identifier with the MOUNT Command

Specifying the volume identifier in the MOUNT command is essential during write operations because it ensures that the correct volume is mounted on the drive and links the continuation volume to the volume set.

Omitting the Volume Identifier with the REPLY/TO Command

To preserve the accessibility character on a volume, you must omit the volume identifier with the REPLY/TO command during a write operation. (When you read from tape, the volume identifier is optional.)

If you initialize and mount a volume set in which each volume has a unique accessibility character that you want to maintain, avoid using the volume identifier because it causes the accessibility character of the first volume in the set to overwrite the accessibility character on the continuation volume.

For example, to preserve the accessibility character, enter the following command in which 3 is the request identification number:

$ REPLY/TO=3

Once the tape file system receives the REPLY command, the system performs checks on the continuation volume to ensure that the volume is the correct one. If it is the correct volume with proper access codes, the system mounts the volume and reissues pending read or write requests to the continuation volume. If the volume fails any of these access checks, the system does not mount the volume (or initialize and mount it in the case of a blank tape).

8.8.3 Modifying Magnetic Tape Characteristics

Use the DCL command SET MAGTAPE to define the default characteristics associated with a specific tape device for subsequent file operations. The SET MAGTAPE command is valid only for magnetic tape devices mounted with foreign volumes.

Use the following format for the command:

SET MAGTAPE device-name

where:

device-name

Specifies the name of the tape device for which the characteristics are to be set. The device must not be currently allocated to any other user.

The following examples illustrate uses of the SET MAGTAPE command in conjunction with the MOUNT command.

Examples

1. $ MOUNT MUB1:/FOREIGN $ SET MAGTAPE MUB1:/DENSITY=800

   
   In this example, the MOUNT command mounts a foreign tape on the MUB1: drive. The SET MAGTAPE command defines the density at 800 bits per inch for writing to the magnetic tape. (The density is reset only if the tape has never been written before.)

2. $ MOUNT MUA0: USER_VOL $ SET MAGTAPE MUA0:/SKIP=FILES:4

   
   In this example, the MOUNT command mounts a tape called USER_VOL on the MUA0: drive. The SET MAGTAPE command directs the I/O subsystem to position the tape to skip four files.
   On local SCSI tape drives, you can use the /FAST_SKIP=option qualifier to skip by file mark or by record. See the OpenVMS DCL Dictionary for more information.

3. $ MOUNT MUA1:/FOREIGN $ SET MAGTAPE/REWIND MUA1:

   
   In this example, the MOUNT command mounts a foreign tape on the MUA1: drive. The SET MAGTAPE command rewinds the volume.

When you finish processing the files or data on a disk or tape volume, use the DISMOUNT command to explicitly dismount a single volume or an entire volume set.

Use the following format when you enter the DISMOUNT command:

DISMOUNT device-name

where:

device-name

Name of the device containing the volume---either a logical name or a physical name. If you specify a physical name, the controller defaults to A and the unit defaults to 0. If the volume currently mounted on the device is a member of a disk or tape volume set, all volumes in the set are dismounted unless you specify the /UNIT qualifier.

You can dismount a volume on a local node or on all the nodes throughout a cluster.

Before dismounting a volume or volume set, the DISMOUNT command checks for conditions that prevent the dismount from completing:

• Installed swap and page files
• Installed images
• Devices spooled to the volume
• Open user file (any files not falling into one of the first three groups)

If none of these conditions is found, the volume is marked for dismount. If any of these conditions exists, the DISMOUNT command does not mark the volume for dismount but, instead, displays error messages indicating the conditions that exist, the number of instances of each condition, and the fact that the volume cannot be dismounted.

If you attempt to dismount the system disk after it has been mounted shared, you may see a message such as the following one, even if there are no user files open:

%DISM-W-CANNOTDMT, AXP27$DKA300: cannot be dismounted %DISM-W-USERFILES, 1 user file open on volume

The message occurs because the file DISMOUNT.EXE is opened as a user file in the course of the dismount operation. To eliminate the error message, install the file DISMOUNT.EXE.

In some cases, you might want to mark a volume for dismount even though files are open on the volume. Marking the volume for dismount prevents users from opening any new files, thereby allowing activity to wind down. You can use the qualifier /OVERRIDE=CHECKS to mark the volume for dismount even if files are open.

Dismounting with Cached Information

As a performance enhancement, the system stores volume information in memory, including information about free space on a disk volume, file identifications, quota file entries, and file headers. This storing of information is called caching. Cached information can include blocks allocated but not yet in a file, or files created but not yet in a directory.

The system writes the information in the caches to the disk when you dismount the disk or shut down the system. If you remove a disk from a drive before the caches are written to disk, the information in the caches is lost. Therefore, you must follow these guidelines:

• Avoid write-locking a volume while it is mounted.
• Do not remove a volume from a drive before it has been dismounted.
• Do not halt the system without performing an orderly shutdown procedure (see Section 4.8.1).

You cannot dismount a volume if any known file lists associated with the volume contain entries. If a volume is referenced in a known file list, you must complete the following steps before you can dismount the volume:

1. Delete all known images associated with the volume using the Install utility DELETE command. For more information, see Section 16.9.14.
2. Wait for:
   1. All processes using those images to release the images.
   2. The system to write writable images back to their files.
      Use the DCL command SHOW DEVICES/FILES to determine the status of the files.

The following sections explain how to perform these tasks:

Task	Section
Dismount a single volume	Section 8.9.1
Dismount a volume set	Section 8.9.2
Dismount foreign volumes	Section 8.9.3
Dismount a volume in a cluster	Section 8.9.4

8.9.1 Dismounting a Single Volume

This section explains procedures to follow in dismounting a single volume and also describes some of the qualifiers you can use with the DISMOUNT command.

8.9.1.1 Dismounting Before Unloading a Volume

Always explicitly dismount a volume or volume set with the DISMOUNT command, or with a command procedure containing that command, before physically unloading that volume. Always wait for the drive to unload before you remove the volume. (You can verify that the dismount is complete by entering the DCL command SHOW DEVICES.)

A private volume is dismounted and unloaded automatically if you log out of the job from which you mounted the volume. If the system fails, however, the drive is not automatically dismounted.

Note that data loss can occur if you do not explicitly dismount a volume and the system fails. For tape volumes, data loss can occur if you unload a volume that contains an open file for which file-trailer labels have not been written. When you remount the volume and attempt to access the file without file-trailer labels, you receive the following error message:

%MTACP-magnetic tape position lost

You can access all the files that precede the file whose file-trailer labels have not been written. However, you cannot access the file that does not have file-trailer labels.

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