The chapter describes what in-file delta technology is and how in-file delta can be used to backup large database files (e.g. a 10GB Outlook.pst file) without uploading the whole database file everyday.

Overview

In-file delta technology is an advanced data block matching algorithm which has the intelligence to pick up changes (delta) of file content between two files when one of the files is not accessible and use the delta information between two files to rebuild one file from the other. Using this algorithm, daily backing up of large file (e.g. a 10GB Outlook.pst file) over low-speed internet connection is made possible because it requires only the changes of information (should be marginal) since last backup to be sent over a low-speed internet connection to complete the backup of a large file (here we assume that the full backup of the file has been saved on the backup server already).
This is what would happen to the backup of a 10GB Outlook.pst file when it is backed up by AhsayOBM with in-file delta technology.

The whole files (10GB), along with its checksum (128-bit) file, are backed up to the backup server. This can be done directly through the internet or indirectly using the seed load utility on a removable hard disk.
When backup runs again later (normally the next day), AhsayOBM will download a checksum listing of all data blocks of the full backup file from the backup server and use it to pick up all changes that have been made to the current Outlook.pst file from the first full backup.
Changes detected are then saved in a delta file which is uploaded to the backup server. (This delta file is assumed to be small because the content of all PST files doesn't change lot of even after it has been updated)
Subsequent backups of this 10GB Outlook.pst file will go through step ii and step iii again. As explained, only a small delta file will be uploaded to the backup server.
With in-file delta technology, daily backing up of large file over low-speed internet connection is now possible

Example 1: If you are adding 200MB to Outlook.pst everyday, the first delta backup will upload a 200MB delta file and the next delta backup will upload a 400MB delta file. This will go on until Day 50 when the delta file required to be backed up reached 10GB. This delta file size (10GB) is now is 50% of the Outlook.pst which is now 20GB (remember that you have added 100MB to this file everyday). If the [Delta Ratio] is set to be 50% (default), the whole Outlook.pst file will be uploaded again.
Example 2: If you are adding 50MB to Outlook.pst everyday, the first delta backup will upload a 50MB delta file and the next delta backup will upload a 100MB delta file. This will go on until Day 100 because it is the [Maximum number of delta] (default) allowed in this backup set and the whole Outlook.pst file will be uploaded again.
All delta files are generated with respect to changes made since the last full backup file (i.e. differential backup). This means that only last full backup file and the last delta file are required to restore the latest snapshot of a backup file. This means that other intermediate delta files are only required if you want to restore other snapshots of a backup file.
In-File delta does differential backup rather than incremental backup. It is designed this way so that a corrupted delta file would only make one particular version of a backup file non-recoverable and all other backups created by other delta files of the same file would still be intact.
The full backup file, its checksum file and the last delta file uploaded (if more than one delta files have been uploaded to the backup server) are always stored in the data area. This means that these files are not affected by the setting of the retention policy and will always be kept on the backup server. This is done this way because all these files are required to get the latest snapshot of the backup file and they should not be removed from the backup server by the retention area cleanup routine. All other intermediate delta files are stored in the retention area.