Technical data

Using BACKUP
11.18 Ensuring Data Integrity
11.18.1 /CRC Qualifier
The /CRC qualifier enables the software cyclic redundancy check (CRC). The
default is /CRC; you must specify /NOCRC to disable checking. Disabling
checking reduces processing time, but increases the risk of data error.
As an output save-set qualifier, /CRC writes the CRC checking code into the
blocks of the output save set.
As an input save-set qualifier, /CRC checks the CRC information in the input
save set.
Compaq recommends that you use the CRC. Although it increases processing
time, it also improves data integrity.
11.18.2 /GROUP_SIZE Qualifier
This output save-set qualifier causes BACKUP to write redundant data to the
output save set. This allows BACKUP to attempt to correct read errors during
the backup restore operation. Use the /GROUP_SIZE qualifier to define the
number of blocks in each group of redundant information. For example:
$ BACKUP/IMAGE/RECORD
_From: DKA100:
_To: MKB100:BACKUP.SAV/LABEL=WKY101/GROUP_SIZE=20
This command adds a recovery block after every 20 blocks of saved data. This
allows BACKUP to recover a corrupted data block for every 20 blocks of saved
data. The value of the /GROUP_SIZE qualifier defaults to 10.
Although using this qualifier increases the size of the save set and the processing
time for the operation, Compaq recommends using the /GROUP_SIZE qualifier to
increase data integrity.
11.18.3 /IGNORE Qualifier
Compaq recommends that you back up your system when no interactive users
are logged in. This is because if BACKUP encounters an open file during a save
operation, it issues an error message and does not copy the file.
You can instruct the backup procedure to save open files by using the
/IGNORE=INTERLOCK qualifier on the BACKUP command. When you use
the /IGNORE=INTERLOCK qualifier, the contents of the file at the moment of
the backup are saved.
The /IGNORE=INTERLOCK qualifier is useful for files that are constantly open
(and would therefore not otherwise be saved). However, you must recognize that
you might be saving inconsistent data, depending on the applications that are
writing to the open files (for example, open application transactions or file data
cached in memory). Also, because of the way BACKUP scans directories, any
activity in a directory (such as creating or deleting files) can cause files to be
excluded from the backup. In general, it is best to back up your system when a
minimum number of files are open.
Also, because of the way the file system works, using the /IGNORE=INTERLOCK
qualifier to back up open files affects subsequent incremental
backups. For example, you can back up an open file with the
BACKUP/IMAGE/RECORD/IGNORE=INTERLOCK command. However, the
backup date field of the file is not updated until you close the file. If the file
remains open during subsequent incremental backups, it is not included in those
backups because its backup date field is not as recent as the last image backup.
11–66 Using BACKUP