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If later backups keep adding far more data than the files you changed, one cause may be how the backup program divides files into blocks. Fixed-size boundaries can shift after a small insertion or deletion, making unchanged data look new. Content-defined chunking (CDC) finds boundaries from the content itself, which can help a deduplicating backup reuse matching portions. It is not a guaranteed size reduction: initial backup size, changed data, compression, and retention also matter.
First, identify which backup size is growing
“Backup size” can refer to several different measurements. A snapshot may represent the full logical contents of your files while adding only a fraction of that amount to storage, because it references chunks already present in the repository. Conversely, the repository’s total size can grow over time as new or changed content accumulates across retained restore points.
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- Source size: the total data selected for backup.
- Bytes scanned or read: data the program examines to decide what changed; this can be large even when little is ultimately stored.
- Bytes transferred: data sent to a remote destination, which may differ from source size because of deduplication, compression, or other processing.
- Newly stored data: content added to the repository for a particular run.
- Total repository size: stored data still needed by all retained snapshots, including unique historical versions.
Check your backup program’s report to see which measure it labels “size,” and compare like with like across runs. Borg describes deduplication across backups and machines that share a repository, but each program reports accounting differently. Borg’s internals documentation explains its repository model.
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How a small edit can make fixed-size backups grow
Fixed boundaries follow offsets
A fixed-size chunker divides a file at regular byte offsets. Imagine a large file divided into blocks, then a few bytes are inserted near its beginning. Every later boundary now falls at a different position. Although much of the original content remains, those later blocks contain different combinations of bytes, so a backup system may be unable to match them with blocks from the previous version.
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This can cause a small edit to lead to substantial new storage in workloads with large, frequently modified files. It is especially worth investigating for virtual-machine images, disk images, databases, and archive files, where modest internal changes can affect content at many offsets. The effect depends on the chunking method and the actual pattern of changes; it does not mean every changed file will be stored in full.
Content-defined boundaries can resynchronize
CDC looks for cut points based on features of the data rather than solely on fixed offsets. A rolling fingerprint examines a moving window; when its value meets the chunking rule, the backup program can end a chunk there. After an insertion or deletion, matching content farther along can produce familiar boundaries again, allowing those chunks to match previously stored data.
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Restic’s design uses Rabin fingerprints with a 64-byte sliding window. Its documentation gives a minimum blob size of 512 KiB, a maximum of 8 MiB, and a target average of 1 MiB. These are restic-specific implementation details, not universal CDC settings. Restic explains the difference between fixed-size and content-defined boundaries in its article on content-defined chunking and its design documentation.
Chunking is only one part of storage growth
Deduplication reuses matching chunks
Chunking decides how content is divided; deduplication checks whether each resulting chunk is already stored. If it is, the backup can refer to the existing chunk instead of saving a second copy. Borg describes repository-wide deduplication, including across machines using the same repository. Its overview says, “Content-defined chunking deduplicates everything in the repository.” That describes Borg’s repository behavior, not a guarantee that every backup program will eliminate all repeated data.
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Deduplication cannot reuse content that is genuinely new or whose chunk boundaries no longer match. A backup can also keep a complete-looking snapshot without storing a complete new copy of every file. See Borg’s data-structure documentation and the Borg project overview for its description of repository-wide deduplication.
Compression changes stored size, not matching
Compression attempts to represent data with fewer bytes; it is separate from identifying repeated chunks. Already-compressed formats, such as many photos, videos, and archive files, may have little room for further compression. Borg documents compression choices with speed and ratio trade-offs; its quickstart describes lz4 as the default in the documented context and lists alternatives such as zstd. Defaults and available choices can vary by Borg release, so check the documentation for the installed version. More intensive compression can consume more CPU.
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Compression and chunking interact in a backup workflow, but choosing stronger compression does not fix shifted fixed-size boundaries. Borg’s quickstart and notes describe its options and trade-offs.
Retention preserves unique history
Keeping more restore points can require more space when those points contain unique versions of files. Deduplication avoids storing repeated chunks again, but it does not erase data needed to restore an older version. Review retention policies and repository accounting separately from chunking; CDC does not make long retention cost-free.
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Diagnose the cause before changing settings
- Read the backup report carefully. Determine whether the surprising number is source data, scanned bytes, transferred bytes, newly stored data, or total repository size. Compare the same metric and time interval between runs.
- Look for large files that change often. Check whether databases, VM or disk images, and archive files are responsible for growth. Their update patterns can make chunking choice consequential.
- Check the chunker and deduplication behavior. Confirm whether your software supports CDC, what its chunk-size settings are, and whether deduplication spans files, snapshots, or machines. Restic and Borg document CDC-based deduplication, but neither documentation establishes which will save more for your particular data.
- Review compression independently. Consider the data types being backed up and the CPU cost of the configured option. Do not assume that another compression pass will substantially shrink media or other already-compressed content.
- Check retention and repository maintenance. Determine how many historical restore points are retained and whether unique old content is still required. Understand pruning and verification procedures before removing snapshots or changing repository settings.
- Test any chunker change safely. Do not casually alter chunker parameters in a populated repository. Different boundaries can cause files touched after the change to be stored again; Borg notes that the effects can accumulate as files are touched and old archives are pruned. Test on a copy or use a fresh repository when appropriate, and confirm that migration and restore plans work before relying on the new configuration.
When CDC is—and is not—the right fit
CDC is most relevant when your workload has large files that change internally and you want matching content after shifted offsets to remain reusable. It is not automatically better for every source. Borg notes fixed-size chunking can be efficient for block devices and raw disk images, so evaluate the actual workload rather than treating CDC as a universal rule.
When comparing tools or configurations, assess the chunking algorithm and adjustable granularity, deduplication scope, compression and CPU trade-offs, encryption and key management, supported source platforms and destinations, restore workflow, verification and pruning behavior, and migration requirements. Borg and restic both document CDC-based deduplication, but there is no general savings percentage or benchmark winner established for every workload. A representative test with your own data is the useful comparison.
Storage capacity does not reduce the footprint
A local disk or USB drive can host a backup repository; Borg lists local and remote destinations in its official project information. An external drive provides somewhere to store backup data and may be useful for a local or offline copy, but adding capacity does not perform CDC or reduce the bytes your backup retains.
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