Is 2 TB of storage for a PC overkill

SSD cache on Synology DiskStation: useful feature or complete overkill?

As the owner of a Synology DiskStation from the more professional sector and in a higher price range, you have probably stumbled upon the "SSD Cache" feature. This is not just a fast intermediate and buffer memory on which frequently used data is stored so that it can be called up in no time at all. The so-called write cache is also about being able to write data as quickly as possible so that it only ends up on the SSDs and only later on the slower HDDs. A great gimmick in theory, but in practice the average person will not benefit from it in every case. Nevertheless, we will show you how you can use the SSD cache on your NAS sensibly and what you need to consider.

Inventory: Can a cache speed up your applications?

Before you start to order a handful of SSDs and use them to build your valuable drive bays, you should think about how you mainly use your NAS. SSD caches only play to their full potential in certain areas, such as:

  • lots of small files
  • many simultaneously connected users
  • Web servers
  • Databases
  • Email servers
  • virtual machines, if applicable
  • as well as extensive write operations where every second counts

For a completely conventional data store and a few services, you will in all likelihood get along quite well without any cache. A file server with mostly large files (music or movies) will hardly benefit from this feature.

However, if you are not the only user on the DiskStation or if you use the NAS as a server for all imaginable things, and you also have two drive bays or NVMe slots free, an installed SSD cache can bring a significant performance boost.

The SSD cache in detail

Synology's Disk Station Manager logs every file access while the cache is running and keeps statistics on which data is accessed and how often. Frequently used files are automatically copied to the read cache here so that they can be accessed more quickly. This all happens in the background and you as a user usually don't notice anything. Unfortunately, you cannot specifically influence which data is made available via the SSD. If you are still curious, you can get more detailed insights from the SSD cache advisor. To do this, open the storage manager, navigate to the "SSD cache" area

Click on the button "SSD cache advisor" and select the corresponding volume whose access statistics you want to view. Select "Next" and confirm the message about the calculation time with "Yes". Depending on the number of files, this process can take a few minutes or even a few hours.

Once the analysis is complete, you will receive a recommendation on the size of your SSD cache. You can also see which amounts of data are used constantly (hot), often (warm), rarely (cold) or not at all (archived).

With write caching, the situation is roughly the opposite. Here, the amounts of data that you copy to the NAS end up on the write cache SSD first. This has the advantage that you can send data to your DiskStation at a rapid pace (until the cache is full). Only over time, and when the NAS is less stressed, will these be written to the slower HDDs and the write cache cleared.

Hardware requirements

In order for you to benefit from the caching feature and to make it work properly, you should take the following to heart:

  • Each volume needs its own SSD cache. You cannot use the same cache for multiple volumes at the same time.
  • One or two more drive or NVMe bays are required, in which you can no longer install hard drives. When buying a NAS, it is best to plan more slots than you actually need at the moment.
  • 416 kilobytes of main memory are used per gigabyte of cache. Doesn't sound like much, but RAM is known to be a limited commodity, especially with DiskStations.
  • Your DiskStation must support the SSD cache function. Most of the plus versions and the models for the professional sector are therefore suitable.
  • With a 10 Gigabit LAN connection, you benefit more from the SSD cache. Many larger models can easily be equipped with a corresponding network adapter.
  • You can use a maximum of 930 gigabytes of cache, even if the installed drives exceed this capacity.
  • Due to the large amount of data written, the lifespan of the SSDs is shortened. You should check their status in the storage manager frequently.
  • When using a write cache, your NAS should be connected to a UPS to prevent data loss in the event of a power failure.

SSD cache for everyone?

Having a cache on the DiskStation offers many advantages when used properly. For example, thumbnails of pictures and videos are also loaded more quickly. In addition, the media server (such as Plex) creates hundreds of thousands of small files that must also be loaded when searching the film or music database. In short, an SSD cache affects most applications and can accelerate them, even if only slightly in some cases. However, you will notice an increase in speed in many areas.

So if you have space for one or two SSDs in your DiskStation, you can safely equip them. Even with small drives (64 gigabytes or less) you can achieve an (often) huge increase in performance at low cost.

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