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Choose Duplicati’s block size for how your files change, remote volume size for the balance between transfer overhead and restore downloads, and retention for the history you need to recover. They control different parts of a backup, so tune them separately. Most importantly, decide on block size before the first backup: Duplicati’s documentation says it cannot be changed after that initial run.
What each setting controls
| Setting | Controls | Main trade-off |
|---|---|---|
| Block size | How source files are split into blocks. | Smaller blocks can capture changes more granularly, but increase the number of blocks Duplicati must track. |
| Remote volume size | The size of files Duplicati places on the remote destination. | Larger volumes reduce per-file overhead, but a restore may need to download a whole volume to recover a small file. |
| Retention | Which historical backup versions remain available. | Keeping more history supports recovery from older changes, but uses more storage; removing versions does not always reclaim remote space immediately. |
These controls are related to backup size, but they are not interchangeable. Duplicati’s backup size parameters guide explains block and volume sizing; its retention settings guide covers version cleanup.
Choose block size for your files and change patterns
Smaller blocks can help when only portions of large files change: changes can be represented in smaller units, potentially reducing how much changed data needs uploading. The trade-off is that Duplicati tracks more blocks, which can add overhead and reduce overall performance. If files mostly stay unchanged or change as whole files, smaller blocks may add tracking work without much deduplication benefit. Duplicati notes that deduplication commonly occurs within identical files or large files whose parts are updated.
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Duplicati’s documentation gives a typical range from about 100 KiB for backups under 3 GiB up to 1 MiB. These are contextual guidelines, not a benchmark or a universal optimum. Consider both the source data size and how it changes rather than selecting the smallest available value by default.
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Set this before the first backup. Duplicati states that block size is fixed after the initial backup because blocks of different sizes cannot be compared. If you need a different block size after that point, treat it as a new backup configuration rather than casually changing the setting on an existing backup set. The value shown by your installed version is the one to confirm during setup.
Choose remote volume size for transfers and restores
Remote volume size determines the size of files uploaded to the destination. Larger volumes mean fewer remote files and less per-file overhead, including headers, connection handshakes, and the work of handling a larger list of files. The cost appears during restore: Duplicati may need to download a full volume to restore even a small file, and files needed for a recovery can be spread across several volumes.
Duplicati’s sizing guide suggests around 50–500 MiB for cloud destinations and 500–2000 MiB for local networks. Treat these as starting ranges, not guaranteed best settings: source size, connection speed, destination behavior, and the kinds of restores you expect all matter.
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- Lean toward smaller volumes when recovering small files or limiting the amount downloaded for an individual restore is important.
- Lean toward larger volumes when reducing remote-file and transfer overhead matters more, and your connection can move larger units efficiently.
The setup guide describes the default volume size as balanced for cloud storage and limited network connections, and advises increasing it for fast connections or local networks. The community advanced-options documentation lists --dblock-size = 50mb; the same page lists a 100kb block-size default. Those are documented option defaults without a release number shown, so check the installed version rather than assuming they apply to every release. The setup guide’s balanced-default description and the advanced-options value are different kinds of guidance, not a release-specific guarantee.
Set retention around the recovery history you need
Pick a rule based on how backups run and how far back you may need to recover:
- Age-based retention: Use it when backups run regularly and a clear time horizon—such as keeping history for a defined period—is the main requirement.
- Version-count retention: Use it when backups are irregular or triggered by external events, so the number of recovery points matters more than their age.
- Bucket-based retention: Use it when you want many recent recovery points but fewer points as they get older.
Duplicati’s setup page presents Smart backup retention as a useful default for most users. Its documented policy, 1W:1D,4W:1W,12M:1M, keeps one backup per day over the most recent week, one per week over the next four weeks, and one per month over the last twelve months. Check that this history actually covers the recovery period you need; a recommended starting policy is not a substitute for that decision.
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Understand what happens to storage after old versions expire
When retention is applied after a backup, Duplicati removes references to versions that no longer qualify. It will not delete the last remaining version. But remote storage may not shrink in proportion to the versions removed: a volume can contain blocks shared by both retained and obsolete versions.
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A practical way to make the choices
- Before the first run, decide block size. Estimate the source size and identify whether large files change in portions or mostly as whole files. Use Duplicati’s documented range as context, then keep the choice fixed for that backup set.
- Choose a remote volume size for your destination and restore needs. Start with the documented range for cloud or local-network destinations, then weigh fewer remote files against the amount you may need to download when restoring small files.
- Choose retention for your recovery timeline and backup cadence. Use age-based rules for regular schedules, version counts for irregular runs, or bucket-based rules for dense recent history and sparser older history.
- Account for cleanup separately from retention. If old versions are removed but storage remains occupied by shared blocks, compaction may be needed to reclaim remote space.
The official documentation provides starting guidance, not one configuration that fits every backup. Your source size, file-change pattern, restore profile, network, and desired history determine the useful combination.
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