Updated: 11 December 1998 |
OpenVMS System Manager's Manual
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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.
To explicitly create a volume set with three volumes, for example, follow these steps:
$ ALLOCATE MUA0: %DCL-I-ALLOC, _MARS$MUA0: allocated $ ALLOCATE MUA1: %DCL-I-ALLOC, _MARS$MUA1: allocated $ ALLOCATE MUA2: %DCL-I-ALLOC, _MARS$MUA2: allocated |
$ INITIALIZE/DENSITY=1600/PROTECTION=(G:RW) MUA0: TAPE1 $ INITIALIZE/DENSITY=1600/PROTECTION=(G:RW) MUA1: TAPE2 $ INITIALIZE/DENSITY=1600/PROTECTION=(G:RW) MUA2: TAPE3 |
$ 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: |
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:
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:
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.
$ MOUNT MUA0:,MUA1:,MUA2: TAPE |
$ INITIALIZE MUA0: MAIN $ MOUNT/OVERRIDE=IDENTIFICATION/INITIALIZE=CONTINUATION MUA0:,MUA1: |
$ MOUNT MUA0:,MUA1: SUN |
$ MOUNT MUA0:,MUA1: SUN,MOON |
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.
$ 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.
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 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:
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:
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.
$ MOUNT MUB1:/FOREIGN $ SET MAGTAPE MUB1:/DENSITY=800 |
$ MOUNT MUA0: USER_VOL $ SET MAGTAPE MUA0:/SKIP=FILES:4 |
$ MOUNT MUA1:/FOREIGN $ SET MAGTAPE/REWIND MUA1: |
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:
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 |
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:
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:
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 |
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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