Integrity Features Of The VMS BACKUP Utility
Copyright (c) Digital Equipment Corporation 1991. All rights reserved
COMPONENT: BACKUP Utility OP/SYS: VMS
SOURCE: Digital Customer Support Center
VERSION INFORMATION:
Information Applies To: VMS, All Versions
OVERVIEW:
The BACKUP Utility is used to save and/or copy data. When saving
files, BACKUP is designed to maintain the data so it can be restored
and used reliably.
This article describes the features of BACKUP that allow it to write
data to a save set and recover that data for later use.
Subjects covered in this article are:
o The BACKUP qualifiers /CRC, /GROUP_SIZE and /IGNORE=INTERLOCK
o Media errors
o Files marked NOBACKUP
/CRC QUALIFIER:
A CRC (Cyclic Redundancy Check) is a mathematical calculation
done bit by bit on data transferred from one location to
another. Some modern tape drives automatically generate a CRC
(Cyclic Redundancy Check) when writing data. This provides a
more robust error detection capability than older model drives.
Data copied to tape or disk must pass through various
controllers and busses and potentially back again. The CRC
generated by a tape or disk drive only checks the data after the
media has received it.
The CRC generated by BACKUP allows it to detect a failure when
any of the components that the data passes through fails to
transmit it correctly. Without the CRC generated by BACKUP,
there is no way to detect that the data was written or read
incorrectly. Any data corrupted or lost due to a failure along
the transmission path cannot be recovered. Therefore, the use of
the /NOCRC qualifier is not recommended.
/GROUP_SIZE QUALIFIER:
When BACKUP writes to a saveset, it organizes the data in groups
of blocks. The size of a block of data can be determined with
the /BLOCK_SIZE qualifier. The number of blocks in a group can
be determined with the /GROUP_SIZE qualifier.
The BACKUP Utility maintains a certain amount of redundant user
data within a group of blocks. If a file has to be restored,
this redundant data can be used in place of a block within the
group that is unreadable.
If the default group size of 10 is used, then a recovery block is
written for every 10 blocks of data stored in the saveset. If
one block within that 10 is unreadable because of media or other
errors, it can be recovered from the redundant data.
If a group size of 100 (BACKUP's maximum value) is specified,
only errors in one block of that 100 can be recovered. If two or
more blocks within a group of 100 contain errors, data will be
lost and cannot recovered.
A group size of zero means that no redundancy information is
maintained in the saveset. Therefore, no replacement data is
available for unreadable blocks.
/IGNORE=INTERLOCK QUALIFIER:
This qualifier allows BACKUP to save a file that is currently
opened for write or exclusive access. This can increase
application uptime while still allowing data to be saved.
However, /IGNORE=INTERLOCK should be used with caution.
When using /IGNORE=INTERLOCK, BACKUP only saves the data that
actually exists in a file on disk at the time of processing.
Data that is in a user or system buffer in memory will not be
saved. Data that is being modified while BACKUP is reading the
file may not be saved in a consistent state.
For example, an RMS file is opened for write by several
processes, each with their own local buffers. Modified data is
stored in these buffers until they are flushed and the changes
are written to the file on disk. These changes may not have been
written to the file at the time BACKUP saves it. It is also
possible that only a partial update of the file on disk has
occurred.
If RMS index information about the location of the data within
the file has not been written to disk, the data that is saved
will not be accessible after a restore because there is no
pointer to it.
Since user data and RMS index information is written separately,
the index structure could point to nonexistent records after a
restore.
MEDIA ERRORS:
Since the BACKUP Utility is designed to save as much data as
possible, when media problems occur (such as PARITY and
FORCEDERROR errors), BACKUP will continue to write data to its
output destination. When restoring files that encountered media
errors while being saved, there is no warning message that a
particular file may have questionable data. Therefore, the
various output that logs BACKUP save operations (Batch Log file,
Backup Log file, etc.) should be examined. Any file that
encountered media errors should be noted.
FILES MARKED NOBACKUP:
The BACKUP Utility can save the attributes of a file, including
its storage allocation, without actually saving the contents of
the file. The attributes of large files such as the system PAGE
and SWAP files can be saved without wasting space in an output
saveset. When a file is marked NOBACKUP, only the file header
(which contains the file attributes), is written by BACKUP to the
output saveset. None of the contents of the file are saved.
A file marked NOBACKUP appears to be in the correct form after
being restored. None of the data previously existing within the
file is available. The only files that should be marked NOBACKUP
are ones where the data is not needed.
For volumes that contain essential user data, it may be advisable
to add the NOBACKUP option to the /IGNORE qualifier
(/IGNORE=(NOBACKUP)). This will prevent the accidental loss of
data if a file was inadvertently marked NOBACKUP. The BACKUP
Utility issues an informational message and lists the files it
encounters with the NOBACKUP attribute. By referencing this
list, the DCL command "SET FILE/BACKUP" can be used to restore
the BACKUP attribute for future BACKUP operations.
NOTE: When using more than one option for the /IGNORE qualifier,
enclose all options within the parentheses, separated by
commas. If the same qualifier is used more than once in
a command line, only the last iteration is used. In the
following example, only the NOBACKUP option will be used:
BACKUP/IGNORE=INTERLOCK/LOG/IGNORE=NOBACKUP input output
