Backup with Volume Shadowing

Volume shadowing binds one, two, or three physical disks together logically in a shadow set. When a write takes place to one disk in the shadow set, that data is forwarded to all members of the set to ensure data integrity. This is not the same as a backup operation for various reasons. For instance, it does not protect against accidental file deletion or software failures.

The shadow set can be backed up and restored using BACKUP with additional care. To see how this is done, consider the following commands. At boot time, assume the shadow set, DSA1:, is formed with three member disks, as shown:

    $ MOUNT/SYSTEM DSA1:/SHADOW=($1$DUA8:, $1$DUA9:, $1$DUA10:) USER 

At some time later, the manager decides to back up this shadow set. This is done by using the shadow set name, not one of the member's names.

     $ BACKUP/IMAGE/VERIFY/RECORD/IGNORE=INTERLOCK -      _$ DSA1: -      _$ MUA400:FULLBU.SAV/REWIND/LABEL=FULL 

This method has the usual cautions regarding open files described previously. If the manager wants to back up the shadow set without worrying about these restrictions, the shadow set has to be manipulated. One of its members must be removed first. First, the shadow set has to be dissolved.

     $ DISMOUNT/SYSTEM DSA1: 

Next, it is remounted with one of the members removed.

     $ MOUNT/SYSTEM DSA1:/SHADOW=($1$DUA8:, $1$DUA9:) USER 

Now that removed member can be backed up without worrying about open files:

     $ BACKUP/IMAGE/VERIFY/RECORD -      _$ $1$DUA10: -      _$ MUA400:FULLBU.SAV/REWIND/LABEL=FULL 

After the backup is complete, this disk can rejoin the shadow set. Any changes that have taken place to the shadow set during the backup period will be made to $1$DUA10 when it joins.

     $ MOUNT DSA1:/SHADOW=$1$DUA10: