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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 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
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.
Figurev 7-2 Backup Created with Sync Point at the End
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