Hong Life CalculatorsUnderstanding the backup window and bandwidth calculator — concept and principle
A backup window is the period, often overnight, in which a backup job must finish without disturbing daytime work. This calculator turns the amount of data, the length of the window and the expected compression or deduplication into the throughput the backup system must sustain, in megabytes per second, and the network link speed that implies, in gigabits per second.
How it is calculated
- Data actually transferred = data size ÷ compression/dedup ratio.
- Required throughput (MB/s) = data size (bytes) ÷ ratio ÷ (window hours × 3,600) ÷ 10⁶.
- Required link (Gbps) = the same bytes × 8 ÷ seconds ÷ 10⁹.
Sizes use decimal units: 1 TB = 10¹² bytes, 1 MB = 10⁶ bytes, and 1 byte = 8 bits. A ratio of 2 means the data shrinks to half before it is sent.
The principle
Throughput is simply work divided by time. If the job has a fixed deadline, the slowest stage in the chain (source disk reads, the network, the backup server, or the target storage) must move at least this much data every second, on average, for the entire window. Compression and deduplication reduce the bytes that have to travel, but only if they happen at the source; if the data is deduplicated at the target, the full amount still crosses the network. Converting bytes to bits explains why a 1 Gbps link carries at most about 125 MB/s before protocol overhead.
Worked example
- Inputs: 5 TB of data, 8-hour window, ratio 2.
- Data to send: 5 × 10¹² ÷ 2 = 2.5 × 10¹² bytes.
- Window in seconds: 8 × 3,600 = 28,800 s.
- Throughput: 2.5 × 10¹² ÷ 28,800 = 86.8 × 10⁶ bytes/s = 86.81 MB/s.
- Link: 86.81 MB/s × 8 = 694.4 Mbps = 0.694 Gbps.
| Backup window (hours) | Required throughput (MB/s) |
|---|---|
| 2 | 347.2222222 |
| 4 | 173.6111111 |
| 6 | 115.7407407 |
| 8 | 86.80555556 |
| 10 | 69.44444444 |
| 12 | 57.87037037 |
| 16 | 43.40277778 |
How to read the result
The figure is an average with no slack. Real jobs lose time to start-up, catalogue updates, small-file handling and verification, and network protocols add overhead, so planners commonly size the path with a healthy margin above the result. Here 0.69 Gbps is close to the limit of a 1 Gbps link in practice, so a 10 Gbps path, a longer window or incremental backups would be safer.
Common mistakes
- Confusing megabytes per second (MB/s) with megabits per second (Mb/s), a factor of 8.
- Applying the dedup ratio to network traffic when dedup runs on the target.
- Assuming a full backup every night when incrementals move only changed data.
Frequently asked questions
How long does 1 TB take over 1 Gbps?
At the theoretical 125 MB/s, 10¹² ÷ 1.25 × 10⁸ = 8,000 s, about 2.2 hours; in practice expect longer.
What is a typical dedup ratio?
It depends entirely on the data. Repeated full backups of similar virtual machines can dedupe very well, while unique media files barely do. Use the figure your backup software reports for your data.
Does a shorter window always need a faster link?
Yes, the required rate scales with 1 ÷ time: halving the window doubles the throughput needed.
Sources: IEC 80000-13, Quantities and units — Information science and technology · W. Curtis Preston, Backup and Recovery, O'Reilly Media