Ensuring Data Integrity

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HP OpenVMS System Manager's Manual, Volume 1:...

Using BACKUP

Backing Up and Restoring the System Disk

Troubleshooting

Ensuring Data Integrity

BACKUP has several qualifiers for further ensuring the integrity of your backups. HP recommends using these qualifiers if you want to achieve maximum data integrity. This section describes some of the ways you can increase data integrity with BACKUP. For more information about these qualifiers, refer to the HP OpenVMS System Management Utilities Reference Manual.

/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.

HP recommends that you use the CRC. Although it increases processing time, it also improves data integrity.

/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, HP recommends using the /GROUP_SIZE qualifier to increase data integrity.

/IGNORE Qualifier

HP 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.

/LOG Qualifier

Use the /LOG qualifier to the BACKUP command to display the file specification of the files that BACKUP processes during a backup operation. For example, if you are copying files in a directory, you can use the /LOG qualifier to display the file specification of each file copied:

$ BACKUP/LOG
_From: WORK3:[OCONNELL]*.*
_To: WORK1:[OCONNELL.SCRATCH]*.*
%BACKUP-S-CREDIR, created WORK1:[OCONNELL.SCRATCH.COM]
%BACKUP-S-CREATED, created WORK1:[OCONNELL.SCRATCH]DECW$MAIL.DAT;2
%BACKUP-S-CREATED, created WORK1:[OCONNELL.SCRATCH]DECW$SM.LOG;42
%BACKUP-S-CREATED, created WORK1:[OCONNELL.SCRATCH]DECW$SM.LOG;41
.
.
.

/VERIFY Qualifier

Use the /VERIFY qualifier to cause BACKUP to compare the contents of the input and output specifiers after a save, restore, or copy operation. When BACKUP is executing the verification pass, it displays the following message:

%BACKUP-I-STARTVERIFY, starting verification pass

If BACKUP finds differences between the input and output files, it issues an error message.

HP recommends that you use the /VERIFY qualifier. Although it increases processing time, it also improves data integrity.

Backing Up a Save Set Twice Using /VERIFY Qualifier

The problem described in this section applies to TZ87 and TZ88 tape drives and to TZ89 tape drives. If you mount a tape /FOREIGN and then back up files to a save set twice, the second save set reports errors under the following conditions:

The two save sets have the same name.

You use the /VERIFY qualifier with the BACKUP command.

The second BACKUP command has a /BLOCKSIZE of less than 4096 bytes (8*512).

The save set is not sufficiently large, for example:

For BACKUP /BLOCKSIZE= The Files Must Total at Least

4000
6300 blocks¹

3580
5400 blocks

For BACKUP /BLOCKSIZE=	The Files Must Total at Least
4000	6300 blocks¹
3580	5400 blocks

Error messages similar to the following ones are displayed:

%BACKUP-I-STARTVERIFY, starting verification pass
%BACKUP-E-READERR, error reading MKB300:[]SET.SAV;
  -SYSTEM-W-DATAOVERUN, data overrun
%BACKUP-E-INVBLKSIZE, invalid block size in save set
%BACKUP-E-INVRECSIZ, invalid record size in save set
%BACKUP-F-READERRS, excessive error rate reading MKB300:[]SET.SAV;
  -SYSTEM-W-DATAOVERUN, data overrun

Footnotes

1 Rounded to nearest hundred

( Number takes you back )

Backing Up and Restoring the System Disk

Troubleshooting