Detailed Example

The key to understanding TSM backup pricing is understanding how files are stored on the backup media. Tracing the life of a single 1 MB file illustrates the fine points. This example assumes you are using the Basic plan (1 revision, 7-day retention period) and doing nightly backups.

If your initial backup were made on January 1, this 1 MB file becomes the current version. Under the Basic plan, you are always allowed one current version and one previous version. The current version exists until it is replaced by a newer backup. If the current version is never replaced, that single backup would be permanent for all practical purposes. In this case, you would be charged the minimum price as only one backup version has ever existed.

At the other extreme, if your file changed daily, you would have two revisions (the maximum allowed under the Basic profile) at any given time.

Of course, most customers will fall somewhere in the middle. There will be some relatively stable files that are stored once and some that change nightly. Actual tape usage will be determined by daily snapshots and averaged at the end of the month to determine appropriate billing. For ease of explanation, these examples are written in the simplest possible terms (pricing and disk usage representing a very stable set of backups). While considering TSM backups, please remember that real world conditions make it nearly impossible to predict what your actual disk usage and costs will be. We can only provide a rough estimate based on the information you provide.

In this example, the file on your workstation is named foo. Foo exists on the client machine. The current backup of foo will be represented as foo.current. The previous revision of foo will be represented as foo.old.

foo.current exists as of the first TSM backup and will exist until it is replaced. If foo.current is created on January 1 and a new backup is made on January 2, the January 1 version of foo.current becomes foo.old and the January 2 version of foo becomes foo.current. If no other changes are made, you have 7 days to recover foo.old. If foo has not been restored from foo.old at the end of one week, foo.old expires and is no longer available. In this case foo.current will always be available for use since it has not been replaced.

If foo is changed again on January 3, the newest version becomes current. The January 2 version of foo.current is bumped to foo.old, and the January 1 version of foo.old is deleted forever.

There are three important concepts to remember when considering TSM backups. First, the cost is determined by the number of files and megabytes used and the number of files and megabytes are partly determined by the frequency of file modifications. Second, you are allowed only as many revisions as are specified in your TSM backup profile. Third, when discussing revisions, the absence of a previously existing file counts as a revision. This means that deleting a file would be read by the backup software as a revision. The file would not exist on the current backup, but would exist in the previous revision.