HP OpenVMS System Manager's Manual, Volume 1:... |
Managing Storage Media |
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The number of devices you specify directly affects the action taken by the tape file system when processing continuation volumes in a volume set. For example, when the number of devices is greater than the number of volumes, the tape files system requests a continuation volume to be mounted on the first drive from the list that does not have a volume mounted.
When mounting a volume set, make sure that all the volumes in the set contain write rings if the user intends to write to any of the volumes in the set. (If even one of the volumes in the set does not contain a write ring at mount time, all volumes are write-locked; the system is unable to write to any of them.) Load the volumes on the drives that have been allocated and place the drives on line.
The following sections explain how to perform these tasks:
| Task | Section |
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Create a tape
volume set
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Creating a Tape Volume Set
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Mount continuation
volumes in a volume set
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Mounting Continuation Volumes in a Tape Volume Set
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Mount volume sets with automatic
switching disabled
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Disabling Automatic Switching
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Creating a Tape Volume Set ![]()
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:
The commands in this example allocate a drive on which you will load each volume.$ALLOCATE MUA0:%DCL-I-ALLOC, _MARS$MUA0: allocated$ALLOCATE MUA1:%DCL-I-ALLOC, _MARS$MUA1: allocated$ALLOCATE MUA2:%DCL-I-ALLOC, _MARS$MUA2: allocated
The commands in this example initialize the volumes. The commands specify the density and the access protection in addition to the device name and the volume identifier.$INITIALIZE/DENSITY=1600/PROTECTION=(G:RW) MUA0: TAPE1$INITIALIZE/DENSITY=1600/PROTECTION=(G:RW) MUA1: TAPE2$INITIALIZE/DENSITY=1600/PROTECTION=(G:RW) MUA2: TAPE3
The commands in this example mount the volumes. The commands include the device name and volume identifier.$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:
Mounting Continuation Volumes in a Tape Volume
Set ![]()
When
mounting a tape volume set, follow the general procedures described
in
Creating a Tape Volume Set.
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).
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 Protecting Tape Volumes). 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.
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.
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.$MOUNT MUA0:,MUA1:,MUA2: TAPE
This example shows the use of the /INITIALIZE=CONTINUATION qualifier for mounting volume sets. It also shows how the system creates volume identifiers for continuation volumes.$INITIALIZE MUA0: MAIN$MOUNT/OVERRIDE=IDENTIFICATION/INITIALIZE=CONTINUATION MUA0:,MUA1:
In this example, the first volume in the set is labeled SUN and is mounted on the MUA0: drive. The second volume receives the identifier SUN_02 and is mounted on the MUA1: drive. The third volume receives the identifier SUN_03 and is mounted on the MUA0: drive.$MOUNT MUA0:,MUA1: SUN
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.$MOUNT MUA0:,MUA1: SUN,MOON
Disabling Automatic Switching ![]()
If your site prelabels volumes, you must disable automatic
volume switching to avoid overwriting these labels. To explicitly
override automatic volume switching, specify the /NOAUTOMATIC qualifier
when mounting a tape volume. (The default is /AUTOMATIC.) Note that
if you allocate only one drive to the tape volume set, automatic
volume switching is implicitly disabled.
When a user is reading or writing to a magnetic tape and the tape reaches end-of-tape position, the system suspends processing and sends a request to mount the next tape in the volume set. For example:
The user does not see this message and might not realize that another tape is needed to complete the read or write operation.%%%%%%%%%%% OPCOM, 28-MAY-2000 15:23:31.78 %%%%%%%%%%%request 3, from user PLAWMOUNT new relative volume 2 (DW0QT2) on MUA1:
The command in this example tells MOUNT not to supply its own label for the second volume but, instead, to use the ones specified in the MOUNT command.$MOUNT/NOAUTOMATIC MUA0: ABCD,EFGH
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
REPLY Command Qualifiers for Continuation Volumes. For more information about these qualifiers, refer
to the OpenVMS DCL Dictionary
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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:
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).$REPLY/TO=3
Modifying Magnetic Tape Characteristics ![]()
Use
the DCL command SET MAGTAPE to define the default characteristics
associated with a specific tape device for subsequent file operations.
The SET MAGTAPE command is valid only for magnetic tape devices mounted
with foreign volumes.
Use the following format for the command:SET MAGTAPE device-name
where:
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device-name
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Specifies the name of the tape device
for which the characteristics are to be set. The device must not
be currently allocated to any other user.
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The following examples illustrate uses of the SET MAGTAPE command in conjunction with the MOUNT command.
In this example, the MOUNT command mounts a foreign tape on the MUB1: drive. The SET MAGTAPE command defines the density at 800 bits per inch for writing to the magnetic tape. (The density is reset only if the tape has never been written before.)$MOUNT MUB1:/FOREIGN$SET MAGTAPE MUB1:/DENSITY=800
In this example, the MOUNT command mounts a tape called USER_VOL on the MUA0: drive. The SET MAGTAPE command directs the I/O subsystem to position the tape to skip four files.$MOUNT MUA0: USER_VOL$SET MAGTAPE MUA0:/SKIP=FILES:4
In this example, the MOUNT command mounts a foreign tape on the MUA1: drive. The SET MAGTAPE command rewinds the volume.$MOUNT MUA1:/FOREIGN$SET MAGTAPE/REWIND MUA1:
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