Incremental vs. Full Backup
To estimate the total time for a computer backup, it is critical to establish several factors. Most importantly, you must determine how much data there is to be backed up. If this backup is what is referred to as a "full" back up, where every single file on the computer will be included in the backup, the total amount of space used on your hard drive, in gigabytes, is the defining number. Conversely, if this is what is known as an "incremental" backup, where only the files that have been modified since the last full backup are to be included, this number will be more difficult to determine.
Other factors that must be taken into consideration when estimating the total time required for a backup is the media that the information will be backed up to. A backup that is being written to a recordable CD or DVD drive will extend significantly the length of time the backup will take to complete as compared to backing up more than a gigabit network to another high speed hard drive. To a lesser extent, the speed of the computer system that is performing the backup, including the type of hard drive installed in the system, will also affect the backup time.
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Real World Performance
Calculating a backup has several variables, but a reliable estimate can be arrived at using an internal hard drive to hard drive architecture as high speed hard drives can transfer data at a rate of approximately 1 gigabyte per 1 minute. Hence, using the drive-to-drive method, a full backup of a computer with 100 gigabytes of data should take roughly between 1 1/2 to 2 hours. This number, however, is theoretically "best case" scenario that a full backup of this size could be completed in and is unlikely to be experienced in a real world environment. A more realistic time metric would be to use a factor of 1 gigabyte of data taking 3 minutes to backup which would increase that 100 gigabyte full backup to approximately 5 hours. It should be noted that using a second hard drive in your system internally is not a reliable method of securing data, because of theft or a catastrophic fire. This method is provided as a best-case scenario to work from and is not a recommendation as a way to secure data.
It has become increasingly popular to backup and store files remotely by using an Internet connection as the transportation vehicle. In this case, the determining factor becomes the upload speed of the Internet connection itself. As a specific example, a T1 line is theoretically capable of transporting 1.5 Mbps (Megabits Per Second) with a megabit being 1/8th of a megabyte. In this example, if we were to calculate the total backup time of a gigabyte's worth of data across a T1 line, the calculation would be: 1 gigabyte = 8 gigabits 8 gigabits = 8,000 megabits 8,000 megabits/1.5Mbps = 5,335 seconds 5,335 seconds/60 = 89 minutes 89 minutes/60 = 1.5 hours.