Synology C2: Tier 2 Data Deduplication


White clouds in a blue skyI’ve been backing up to the Synology C2 Backup service since it became available worldwide. For the first month, I used the Tier I plan. This tier comes with low prices, but it also has some limitations. I then looked at the Tier II plan, to see what benefits it brought in real life. The two significant differences (besides price) are unlimited backup version and data deduplication.

The price of unlimited backup versions is that all those versions count against your quota. Whereas, the Tier I plan only counts size of the size of the files that are on your NAS and ignores the backup versions. Synology promotes data deduplication as a way to save the space used. They also say “However, the Plan II-exclusive deduplication can help you reduce duplicated data across different backup versions, thus optimizing the storage usage.” But, is this really the case? In my testing, I found data deduplication does not save space across backup versions.

The Tier II plan offers data deduplication which might reduce the space used on the C2 service, and therefore how much you can back up for your money. While there are indeed specific cases where data deduplication on Synology C2 can save significant space, for typical home and small business users it won’t help very much.

If your thinking about switching to a Tier II plan for Synology C2 Backup, these are the things to consider.

HyperBackup does file-level data deduplication

According to the documentation, HyperBackup does file-level deduplication. To be deduplicated, the file contents must be the same. These means a small change in any file will result in a new file being backed up and the old file saved as a backup version. I confirmed this in my testing.

HyperBackup does not deduplicate across backup tasks

Files are not deduplicated between two Synology NAS devices, or even two different backup tasks configured on the same device. I did confirm this with testing, with and without using encryption.

So if you want to backup your primary NAS along with a backup that has the same files, you won’t be saving any space. To maximize deduplication on the Synology NAS, you’ll want everything backed up to Synology C2 in a single HyperBackup task.

HyperBackup does deduplicate identical files with different names or attributes.

In my testing, I renamed some files, and they were deduplicated correctly. The amount of storage used did not increase when I copied, then renamed 12 GB worth of files and backed them up. (There is some overhead, my storage usage increased about 50 MB.)

I also touched that 12 GB worth of files to change the modified date (this doesn’t change the file contents). and the files were properly de-duped. In this case, the old files are now a backup version, and the new files are the current backup.

There may be cases where data deduplication fails

In one of my tests, not all the files were properly deduplicated. In one test I had five identical 2.4 GB files, but with different names. After the backup, there was 8 GB of additional space was used on Synology C2, which indicates only a couple of the files were deduplicated.

To verify the files could be deduced I then backed up one of the files on its own, and then the other 4 in a second backup. In this case, all four additional coped were deduplicated.

Data deduplication takes place on the client

HyperBackup will do the deduplication before sending the file to Synology C2, which has the benefit of saving both bandwidth and time.


A Synology C2 Backup Tier II plan allows more flexibility in your backup strategy. You can keep backup versions for years, or one version per day for however long you want. But, that flexibility needs to be weighed against the added cost in space used. By their nature, the backup versions are different files and therefore won’t benefit from deduplication.

Deduplication will be of limited benefit in many situations. Unless you frequently have the same file scattered around different locations on your Synology NAS, there won’t be much space savings.

While there are reasons for wanting a Tier II plan, deduplication will not save you space over a Tier I plan if you will be keeping a lot of backup versions. You can adjust your backup rotation, but if you’re too liberal in your scheduling, you could hit the 1GB quota and be required to pay an additional €69 for the 2GB plan.

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