Backup window and RPO fit calculator
Does the backup fit the overnight window? Is the link or the target disk the limit? Does a weekly full plus daily incremental meet the RPO target? Enter volume, change and speeds to see backup time, the bottleneck and, if it does not fit, the speed or the number of splits you need.
Results
- Full backup · Duration
- 12.7 h
- Data size (logical): 8 TB · Written to disk (reduced): 4 TB · Bottleneck: Link
- Full backup · Fits the window
- Fits
- Window utilisation: 26.5 % (2 d)
- Incremental backup · Duration
- 23 min
- Data size (logical): 240 GB · Written to disk (reduced): 120 GB · Bottleneck: Link
- Incremental backup · Fits the window
- Fits
- Window utilisation: 4.8 % (8 h)
- Worst-case RPO
- 24.4 h
- Does not fit (Target RPO: 24 h) · Interval 24 h + duration of the last backup 23 min
Restore chain
- Backup sets needed
- 7
- Data to write back (logical)
- 9.44 TB
- Estimated write-back time
- 30 h
- Data written per cycle
- 4.72 TB
Data transfer only; there is no allowance for rebuild, validation or application start-up. For RTO use the RPO/RTO tool.
Total written to disk (reduced) over one full-backup cycle (7 days)
If it does not fit
RPO fit
- To meet the RPO target the backup interval must be at most 23.6 h.
- The best fit among the options: 12 h.
Calculation and units
- Units are decimal: 1 GB = 10^9 bytes, 1 TB = 10^12 bytes, 1 Mbit/s = 10^6 bit/s, 1 MB/s = 8 Mbit/s.
- Written to disk = logical data ÷ reduction ratio. Time = slowest link: max(link data ÷ (link speed × efficiency), written to disk ÷ disk speed).
- Incremental = volume × daily change × interval/24 (capped at the volume). Differential: change accumulated over k × interval at run k. Runs per cycle = full interval ÷ interval − 1.
- Worst-case RPO = run interval + duration of the last backup. Restore = full + all incrementals in the chain (only the last for differential), unreduced data ÷ restore speed.
This is a preliminary estimate. Real times depend on your backup software, data type (database, virtual machine, files), number of small files, concurrent jobs and the read speed of the source system. Changed blocks are assumed not to change again; incremental and differential sizes sit near an upper bound. Runs are assumed to start at the beginning of each interval. Test backup and restore in your own environment.
Restore time (RTO), storage for retention and the 3-2-1-1-0 check are in a separate tool: Backup RPO/RTO calculator
Shall we review your backup design, your window and your restore scenarios together?
Request a meeting01
How to use
A
Enter the data volume, daily change rate and compression/deduplication ratio; choose whether the reduction happens at the source or at the target.
B
Enter the link speed, the efficiency and the sustained write speed of the target disk; choose the strategy (full + incremental, full + differential or full only) and the frequencies.
C
Enter the daily and full backup windows and, optionally, the target RPO; see the durations, window fit, bottleneck, restore chain and, if it does not fit, the speed or number of splits you need.
02
How is the backup window calculated?
Backup time is the time of the slowest link. Data first crosses the link and is then written to the target disk; link speed is multiplied by efficiency (protocol, encryption and sharing losses), while disk speed is the sustained write speed. If the reduction happens at the source, the link carries the reduced data too; if deduplication happens at the target, the link carries full data and only the disk writes the reduced amount. The tool calculates both times, takes the larger and shows the bottleneck.
The result is compared with the backup window. Giving the full backup and the incremental/differential their own windows matches real setups: the full backup usually runs in a long weekend window and the incremental in a short night window. For RPO fit the worst case is the run interval plus the duration of the last backup; the tool compares the interval with the target and finds the largest interval that meets it.
03
Full, incremental and differential: the effect on the window
A full backup takes all the data, so it takes longest. An incremental backup takes what changed since the last backup; it is small and quick, but a restore needs the full backup plus every incremental in the chain, so the longer the chain, the longer the restore and the greater the risk that one link in the chain is damaged. A differential backup takes what changed since the last full; it grows as the cycle goes on, but a restore needs only the full and the latest differential.
As the interval shortens each run gets smaller but there are more runs. If a run takes longer than the interval the backups overlap and RPO becomes worse than the interval; the tool shows this as a warning. In high-volume environments the full backup may not fit the weekly window; the remedy is usually to split the data into groups and take a full backup of one group each night, or to use synthetic full backups.
04
What to do if the window is too small
Look at the bottleneck first: if the link is the limit, raise the link speed, move the reduction to the source or put backup traffic on a separate network; if the disk is the limit, you need a faster target (more spindles, SSD cache, scale-out storage). The tool calculates the link and disk speed needed to fit the window and the ratio needed to fit by reduction alone.
If speed cannot be raised, splitting the data, rolling full backups, synthetic full backups or forever-incremental methods shrink the window. If the RPO is hours or minutes, snapshots and replication suit better than backup. In industrial environments (historian, SCADA servers) the window should follow the line schedule so that backup I/O does not affect production and control loops. For RTO, storage for retention and the 3-2-1-1-0 check, use the Backup RPO/RTO calculator.
FAQ
- Are the values I enter sent anywhere?
- No. The calculation runs entirely in code in your browser; values are not sent to any server and are not stored.
- How is this tool different from the Backup RPO/RTO tool?
- The RPO/RTO tool calculates restore time (RTO), storage for retention, WAN seeding and the 3-2-1-1-0 check. This tool focuses only on the backup window: the duration of full and incremental backups, window fit, bottleneck, RPO fit, the restore chain and the speed you need if it does not fit.
- What is the difference between differential and incremental backup?
- An incremental backup takes what changed since the last backup (full or incremental); a restore needs the full backup and every incremental in the chain. A differential backup takes what changed since the last full; it keeps growing, but a restore needs only the full and the latest differential.
- Is the speed limited by the link or by the disk?
- By both: backup time is the time of the slowest link. The tool calculates the link time (efficiency included) and the disk time separately, uses the larger and shows the bottleneck.
- Should I reduce at the source or at the target?
- Source-side compression/deduplication reduces the data on the link, which makes a big difference on narrow connections, but adds CPU load on the source server. Target-side deduplication spares the source server, but the link still carries full data. The tool's option lets you compare the two cases.
- When can an RPO target not be met by backup?
- If the backup duration alone exceeds the target, shortening the interval is not enough. For targets in the range of minutes you need continuous protection such as snapshots and replication.
Make your backup fit its window
Let us design the backup architecture, windows, link and disk capacity and restore scenarios together. In a free discovery call we discuss your current setup and targets.