STORE and TurboSTORE/iX Products Manual (B5151-90003)

Chapter 7 125
TurboSTORE/iX 7x24 True-Online Backup
How Shadow Logging Works
How Shadow Logging Works
TurboSTORE/iX 7x24 True-Online Backup uses shadow logging to intercept any changes
made to files while they are being stored.
When the Sync Point is at the Beginning
When the sync point occurs at the beginning of the backup (ONLINE=START), shadow
logging guarantees that TurboSTORE captures the state of the fileset as it is right before
the files begin to be written out. When a file is opened for write access, any changes made
to the file are monitored by shadow logging. When a modification occurs, the part of the file
being changed is copied. This "before image" is added to internal shadow log files. Once the
data has been copied, the modification can proceed. The before image logs for all files are
maintained until the information is requested by TurboSTORE. TurboSTORE requests a
file's log data once it has finished storing the file. The log data is written immediately
following the file data. The before image log data is maintained until the media that the
file resides on has been completed. At reel switch time, or at the end of the backup
(whichever comes first), the before image log data for all files on that media is released.
By default, the before image log files are created on the same volume set as the files being
stored. You can create the log files on a specific volume set by specifying the volume set
name using the LOGVOLSET option, as explained in "Performing Online Backup" in Chapter
6, "STORE and TurboSTORE Command Options."
RESTORE restores a file by first reading the file data that had been written to the media.
The log data is then read in and replaces any sections of the file that were already
restored. Once a file's before image logs are applied, the file is restored to its state at the
time of the sync point.
Figure 7-1 shows what a backup tape looks like if the sync point occurs at the beginning of
the backup (ONLINE=START).
Figure 7-1. Backup Created with Sync Point at the Beginning