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

126 Chapter7
TurboSTORE/iX 7x24 True-Online Backup
How Shadow Logging Works
When the Sync Point is at the End
When the sync point occurs at the end of the backup (ONLINE=END), shadow logging
captures the state of the files as they are at the end of the backup. In this case, instead of
copying the before images of modifications to log files, shadow logging uses a bitmap to
keep track of the modified pages. Once the backup is completed, the bitmap is used to
collect the modified "after image" pages from the system. This data is then written out to
the end of the media. For parallel backups, after image log data is written at the end of
each parallel set. For more information on parallel backups, see Chapter 3, "Preparing
Storage Devices".
During the backup when ONLINE=END is specified, logging is turned on for a file when
TurboSTORE begins to write it out. The logging stays on for a file until the sync point
occurs at the end of the backup.
When a file with after image log data is restored, the initial file data is read in first. Once
the after image log data is read in, it is applied to the file data, reapplying all changes
made to the files during the backup. This restores all files to their state at the end of the
backup.
However, since the log data is at the end of the backup, files with after image (AI) data are
not available until the Restore completes and the after image log data is applied.
Figure 7-2 shows what a backup tape looks like if the sync point occurs at the end of the
backup.
Figure 7-2. Backup Created with Sync Point at the End