A NAS backup can use more space than the data currently on your drives because it may preserve older versions of files, retain blocks referenced by snapshots, and include restore or system data in its storage total. Deduplication and compression can reduce the space used, so the result depends on the backup software, what changes, how long versions are kept, and how the NAS accounts for storage.
Why can a NAS backup take more space than the current source data?
A backup destination is not always a mirror of the source’s present state. If it keeps restore points, it may also store data that has since been changed or deleted. Snapshots can preserve blocks that would otherwise be reclaimable, and some products include restore-related or system space in their reported total.
For Synology Hyper Backup, the vendor says the destination can also be smaller than the source because of deduplication and compression. These effects vary with the data and configuration; there is no universal ratio that predicts the result for every NAS. See Synology’s Hyper Backup documentation and its explanation of differences between source and destination backup size.
What makes backup storage grow?
Retained versions keep historical data
When a file changes, a versioned backup may preserve its earlier state so you can restore it. When a file is deleted from the source, a retained restore point may still need that file. The destination therefore represents a history of states, not merely the current contents of the drives. Synology’s ActiveProtect Manager guidance puts it this way: “Deleting data from the source does not immediately reduce the value because the deleted data may still be required by retained backup versions.” This describes that product’s accounting, not every NAS’s reporting method. See Synology’s explanation of ActiveProtect Manager data sizes.
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Snapshots can keep blocks in use
Backup versions may share deduplicated blocks. Removing one version does not necessarily release a block if another retained version still refers to it. A snapshot of a backup folder can also retain data independently of the backup-version cleanup, so deleting versions may not promptly reduce the volume’s reported use. Synology describes this behavior for Active Backup for Business in its article about deleted versions that do not free NAS space.
Snapshots can also affect efficiency processing. Synology’s current guidance recommends running deduplication or compression before taking snapshots, because data referenced by a snapshot cannot be processed by those tasks. Feature support depends on DSM version; Synology says DSM 7.4 and later supports both deduplication and compression. See its Snapshot Replication and storage-efficiency guidance.
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Product totals may include more than backup data
Storage figures are not always measuring the same thing. In ActiveProtect Manager, physical backup data is distinct from total storage usage; the total also includes restore-related data, reserved system space, and other system data. Those are product-specific definitions. Check whether your NAS is reporting logical source data, physical backup data, or total storage before comparing figures. Synology details its terms in the ActiveProtect Manager data-size guide.
Why might the backup be smaller than the source?
Deduplication avoids storing duplicate data more than once, while compression reduces the size of stored data. Synology documents both capabilities for Hyper Backup and notes that compression is configured when a backup task is created. The savings depend on file types, duplicate content, and settings; already-compressed or unique data may yield less reduction. Do not treat a particular reduction percentage as guaranteed.
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How should you estimate the capacity you need?
Synology publishes this estimate for its Active Backup suite:
Estimated storage requirement = total backup data × (1 − deduplication rate) × (1 + daily data change rate × retained versions)
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The inputs represent the data being backed up, the assumed deduplication rate, the daily change rate, and the number of retained versions. Synology’s examples use telemetry-based assumptions and warn that results may vary by organization. For physical or virtual devices, its 2026 small-scale example uses a 5% daily change rate, 60 retained versions, and 50% deduplication; its large-scale example uses 7%, 60 versions, and 50% deduplication. These are Active Backup suite examples—not universal NAS constants or a promise of compression for an individual workload. See Synology’s Active Backup storage-sizing guidance.
Use the estimate as a starting point, then compare it with actual destination usage as backups accumulate. Your measured change rate, retention settings, snapshot policy, and the product’s accounting rules matter more than adopting a vendor example unchanged.
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What should you check when backup usage seems unexpectedly high?
- Check how many restore points are retained. Review the backup task’s rotation or retention policy and whether daily, weekly, monthly, or yearly versions are kept. Hyper Backup’s settings documentation explains its backup settings.
- Check snapshots of the backup destination. Determine whether snapshots retain older states even after backup versions have been deleted.
- Identify the storage figure being compared. Establish whether the source number is current logical data and whether the destination number is physical backup data or a total that includes restore and system space.
- Review deduplication and compression settings. Confirm which efficiency features are supported and enabled for your NAS, DSM version, and backup task.
- Allow for shared data and cleanup behavior. A removed version may share blocks with other versions, and snapshots may still reference data; do not expect a one-for-one immediate capacity reduction.
How to compare backup options fairly
When evaluating two backup configurations, compare the same workload and look at the factors that determine both recovery and space use:
- Retention: how many restore points are kept and for how long.
- Storage efficiency: whether and how deduplication and compression are applied.
- Snapshot interaction: whether snapshots retain blocks or constrain efficiency processing.
- Capacity definitions: how the product distinguishes source data, physical backup data, and total storage usage.
There is no single best retention or storage-efficiency setting for every workload. More retained versions can improve recovery options while requiring more capacity; the appropriate balance depends on your recovery needs and measured data changes.
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