Running Out of NAS Space? DSM 7.4 Can Reduce HDD Data
With DSM 7.4, Synology has introduced post-processing deduplication and compression for supported HDD volumes. Instead of requiring an all-flash storage system to benefit from advanced data reduction, compatible HDD-based Synology systems can now analyze existing data, eliminate redundant blocks, and compress eligible information after it has been written.
For businesses storing years of documents, logs, project archives, ISO files, and other repetitive or compressible information, the potential savings can be substantial.
Synology reports HDD data-reduction ratios ranging from 1.5:1 to 3.5:1 in its internal real-world workloads. Actual savings, however, depend heavily on what you store. Understanding how the technology works is therefore essential before enabling it.
What Does Post-Processing Mean?
The important term in DSM 7.4 is post-processing. Synology does not have to deduplicate or compress every eligible block while an application is actively writing it.
Instead, storage-efficiency tasks can process eligible data after it has been stored. That separation is useful because deduplication and compression consume system resources. DSM allows administrators to schedule storage-efficiency operations so they can be performed during appropriate periods rather than unnecessarily competing with important production workloads.
This is particularly relevant for HDD arrays, where businesses may be balancing file services, backups, applications, and other storage activity.
Deduplication and Compression Solve Different Problems
Although they are frequently discussed together, deduplication and compression are not the same thing. Deduplication looks for identical data blocks. Imagine storing multiple copies of similar VM images, ISO files, archived project folders, or repeated business datasets. If identical blocks exist, deduplication can avoid unnecessarily storing those blocks repeatedly.
Compression addresses a different problem. Instead of finding duplicate blocks, it attempts to reduce the amount of physical capacity required to store the information inside eligible blocks. Plain-text documents, logs, and uncompressed databases, for example, may contain relatively little duplicated data while still compressing effectively.
DSM 7.4 therefore allows HDD administrators to choose among three approaches:
- Deduplication and compression
- Deduplication only
- Compression only
The correct choice depends on the workload rather than simply selecting the most aggressive option.
Which Mode Should You Use?
Synology provides useful guidance for selecting a mode.
- Deduplication and compression provides the greatest opportunity for overall reduction and is suited to general archival data or datasets with considerable redundancy.
- Deduplication only makes more sense when the storage contains large amounts of repeated blocks. Synology specifically identifies workloads such as virtual machine images and ISO files as examples.
- Compression only is appropriate when information has relatively little duplication but compresses efficiently. Text files, logs, and uncompressed databases are good examples.
This means two 20 TB Synology systems could achieve dramatically different savings even when both contain exactly 20 TB of logical data. The content matters.
What Do Synology’s 1.5:1 to 3.5:1 Results Actually Mean?
Synology reports HDD data-reduction ratios ranging from 1.5:1 to 3.5:1 across internal workloads including file collaboration, log retention, and document archiving. Those ratios are easier to understand when converted into practical examples. Suppose you have 10 TB of logical data.
At a 1.5:1 reduction ratio, approximately 6.67 TB of physical storage would be required.
At 2:1, approximately 5 TB would be required.
At 3.5:1, the same 10 TB of logical information would occupy approximately 2.86 TB.
| Data Reduction Ratio | 10 TB Logical Data | Approx. Space Saved |
|---|---|---|
| 1.5:1 | 6.67 TB physical | 3.33 TB |
| 2:1 | 5 TB physical | 5 TB |
| 2.5:1 | 4 TB physical | 6 TB |
| 3.5:1 | 2.86 TB physical | 7.14 TB |
These figures illustrate the mathematics of the ratios; they are not a promise that your NAS will achieve those results. Synology explicitly states that savings depend on workload characteristics, data patterns, and system configuration.
How DSM Calculates Your Actual Savings
Fortunately, you do not have to estimate the results yourself after a task runs. DSM compares the logical size of stored information with the physical capacity actually consumed.
For example, suppose a shared folder contains 100 GB of logical data but consumes only 70 GB physically after storage-efficiency processing.
DSM calculates:
100 GB – 70 GB = 30 GB saved
The reported saving would therefore be 30%.
Storage Manager updates these savings statistics after the applicable storage-efficiency task completes. That gives administrators a much better measurement than relying on theoretical compression or deduplication ratios.
HDD Storage Efficiency Is Configured Per Shared Folder
One particularly useful design choice is that DSM 7.4 does not require HDD storage efficiency to apply indiscriminately to everything on a volume. For HDD volumes, deduplication and compression are configured at the shared-folder level. This means an administrator could enable storage efficiency for an archival folder while leaving a performance-sensitive active dataset untouched. That makes it possible to be selective.
For example:
Archive: Deduplication + compression
VM repository: Deduplication
Log repository: Compression
High-performance active data: Neither, if testing indicates that is preferable
DSM can also be configured to automatically apply a selected storage-efficiency mode to future shared folders. Synology consulting services can help select suitable folders and efficiency modes.
Not Every Synology NAS Can Use HDD Data Reduction
Compatibility is one of the biggest limitations. DSM 7.4 does not suddenly provide HDD deduplication and compression to every Synology NAS.
Synology states that HDD data reduction is available on selected models from 2022 onward, and administrators should check their specific model’s software specifications. The storage pool must also consist entirely of supported Synology HDDs.
In addition:
- The volume must use Btrfs
- The volume must be Healthy
- Usage detail analysis must be enabled
- Hardware/model support must be verified
If your NAS uses third-party HDDs or an unsupported Synology model, upgrading to DSM 7.4 alone does not make this feature available.
Some Data Cannot Be Processed
Even on a compatible system, not everything stored on the HDD volume is eligible.
Synology excludes several data types from HDD storage-efficiency processing, including data stored in snapshots, encrypted shared folders, Hybrid Share folders, hot-tier folders participating in a tiering plan, and certain package-created or package-data shared folders.
That can materially affect expected savings. If a large portion of your storage consists of excluded data, headline reduction ratios may have little relationship to your actual results.
Snapshots Require Special Attention
Snapshots create another important consideration. Synology states that storage-efficiency tasks cannot process data referenced by a snapshot.
That means an aggressive snapshot schedule can potentially limit how much data is available for deduplication or compression. Synology recommends running storage-efficiency tasks before creating new snapshots when maximizing capacity savings is important. This does not mean snapshots should be abandoned. Snapshots and data reduction solve completely different problems. Snapshots provide recovery points. Deduplication and compression reduce storage consumption. The goal is to coordinate their schedules so one does not unnecessarily interfere with the other.
Where Does HDD Deduplication Make the Most Sense?
The strongest candidates are generally datasets that are large, relatively cold, and repetitive or compressible.
Examples include:
- Long-term document archives
- Historical project files
- Log repositories
- Repetitive file shares
- ISO collections
- Certain VM-related datasets
- Uncompressed databases
- Compliance archives
- Cold business data
Constantly changing, latency-sensitive production workloads deserve more careful testing.
Synology itself recommends considering performance implications before enabling storage efficiency in performance-sensitive environments.
Don’t Treat a 3.5:1 Ratio as Guaranteed Capacity
The biggest mistake would be purchasing storage under the assumption that a 3.5:1 ratio is guaranteed. It isn’t. A 20 TB dataset consisting primarily of already-compressed video, JPEG photographs, ZIP archives, and other compressed formats may behave very differently from 20 TB of repetitive documents and logs.
Capacity planning should therefore distinguish between:
Logical capacity: the amount of information users and applications believe they are storing.
Physical capacity: the amount of actual drive space consumed after data reduction.
DSM’s Storage Efficiency reporting allows administrators to compare those numbers after processing and determine whether the feature provides meaningful value for their particular workload.
Can DSM 7.4 Delay Your Next Storage Expansion?
For the right workload, potentially. Consider a business approaching the limits of an HDD array because years of document archives, logs, and repetitive project information have accumulated.
If storage efficiency substantially reduces the physical footprint, the organization may reclaim capacity without immediately installing larger drives or expanding to another chassis. Large storage solutions support sustainable growth beyond short-term capacity savings.
But storage efficiency should not replace proper capacity planning. Administrators still need sufficient free capacity for growth, snapshots, system operations, backups, and unexpected workload increases.
The better way to think about DSM 7.4 Storage Efficiency is:
It makes eligible capacity work harder. It doesn’t make capacity planning unnecessary.
Is DSM 7.4 HDD Storage Efficiency Worth Enabling?
If you have a supported Synology system with Synology HDDs and large amounts of cold, archival, repetitive, or compressible Btrfs data, DSM 7.4’s Storage Efficiency feature is worth evaluating. The most important step is not immediately turning on every reduction option. Start by identifying the type of data stored in each shared folder.
If duplicate blocks dominate, evaluate deduplication. If highly compressible information dominates, compression may be sufficient. For general archival datasets containing both, combining the two may deliver the best result. Then measure the outcome in Storage Manager.
About Epis Technology
Epis Technology helps businesses design, expand, and optimize Synology storage environments based on real capacity, performance, and data-protection requirements. Services include DSM upgrades, Synology NAS deployment and support, Btrfs storage architecture, capacity planning, storage-efficiency configuration, HDD and SSD planning, snapshots, Hyper Backup, virtualization storage, Microsoft 365 and Google Workspace backups, ransomware protection, networking, and disaster recovery. Epis Technology can help determine whether DSM 7.4 deduplication and compression are appropriate for your workloads and how much additional usable capacity your existing Synology infrastructure may realistically provide.