Why is more space being used on the backup server than the total space used by the files selected for backup?

The reason why the space used on the backup server is higher than the amount being backed up is that space is used for the incremental backups made within the backup window. If you have a backup window that is 7 days, you will have the space used by a full backup on day 1 PLUS the space used by the incremental backups of any CHANGED files on day 2, 3, 4, 5, 6, and 7. You can lower the amount of space used by shortening the backup window. For example, if you have a "high visibility" system where any problems are brought to your attention immediately, you may be able to get away with having backups that go back only 2 days. i.e. any errors that require restoration of backup files are detected prior to all of the "good backups" being overwritten by junk.

Backups will cease once the disk space quota associated with the account is exceeded. The only way to “restart” the backup process successfully is to free up enough space on the backup server to allow the next backup to occur without exceeding the quota.

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