2. File versions & types of manual backup
On the HFS at Oxford, the TSM server is configured to keep up to two copies of each file; a copy of the latest or current version and a copy of the previous version (if it existed). If a third (or subsequent) copy is made of the file, at the next backup the oldest copy held is deleted, the current one held becomes the oldest and the very latest becomes the current copy. If the file is deleted on the client machine some rules come into play saying how long the two copies will be retained on the HFS server. This depends on whether the client is a desktop/laptop or server - please see our section on data retention. In TSM terminology, the backup copy of the current version of a file is called the active version; the backup of the previous version is called the inactive version.
There are up to four types of manual backup available, some depending on the client operating system; the screen shot below is for the Windows client but is similar to the TSM interface on other client platforms :
- Incremental (complete) - also known as a full incremental backup. In a full incremental backup the server and client compare notes about which files are on the client and which backups are on the HFS to allow a full reconciliation to take place. The server passes full details of the client's filestore to the client and the client compares this with the current filestore; any new or changed files are then backed up. A file is considered to have changed if any of the following properties have changed: size, permissions, owner, modification date/time. The client tells the server which files have been deleted since the last backup; the server then marks the backup copies for deletion. They are not deleted immediately but become inactive and are retained for the period specified for desktop/laptop and server backups (see our section on data retention), whereafter they are deleted from the server and can no longer be restored. On NT, 2000 and XP machines this reconciliation is potentially different if the Journal Engine Service is running on the local partition. In this case the client records changes to files in a local journal database and uses this to determine which files are eligible for backup. This is useful on drive partitions with folders holding very large numbers of files, only a small percentage of which change. A full incremental backup is the normal mode for a weekly backup.
- Incremental (date only) - also known as partial incremental backup. In a partial incremental backup the server passes to the client the date of the last backup of it's filestore partitions. The client then decides which files to backup - essentially all files with a modification date later than this. New files on the client which have a modification date earlier than the date of the last backup of the partition - for example recently installed software files - will not be backed-up by a partial incremental. Similarly, files deleted from the client since the last backup will not be marked inactive at the server. Note that a partial incremental only updates the last backup date for a partition if it completes a backup of the entire partition. If a partial incremental is performed on only part of a filesystem partition, the date of the last backup is not updated and the next partial incremental will back up these files again. An incremental by date backup is much quicker than a full incremental and is the recommended mode for extra backups carried out between the weekly full incremental backup.
- Incremental (without journal). This only applies to NT, 2000 and XP clients with the Journal Engine Service switched on. In this case the client performs a standard full incremental backup as above, and bypasses the journal database. This is useful to resync the journal and the server when, for example, the journal is corrupt or the Journal Engine Service is stopped or started for any reason.
- Always backup - also known as selective backup. In a selective backup the client backs up the files and directories chosen, unconditionally. This means that all file(s) will be backed up irrespective of whether they have changed since the last backup. This can potentially be very useful if you want to be sure that you have a backup of a particular set of files that you are working on. The disadvantage is that by running an 'Always' backup on a file which is already backed up is that it results in holding two identical versions of that file. In contrast, full and partial incremental backups only backup a file again if it has changed, thereby ensuring that the active and inactive versions are different. This is not a backup mode that we would unreservedly recommend.