SSD Cache

SSD cache improves the performance of random access by storing frequently accessed small files (randomly placed small blocks of data) on the SSDs of an SSD cache mounted on a volume or LUN.

The objective of enabling SSD cache is to increase the performance of random access to a small portion of data that is frequently accessed in the storage space. For example, both large sequential read or write operations (e.g., HD video streaming) and entirely random data reading patterns lack re-reading patterns, and thus will not benefit significantly from SSD caching. For general applications, it is recommended to enable the Skip sequential I/O option, so that sequential I/O can still pass through to the drives of the storage space.

Note:

SSD Cache Types

There are two types of SSD cache, both implementing an LRU (Least Recently Used) algorithm to swap the data in the cache:

Note:

Memory Requirements

SSD caches require a certain amount of system memory. The larger the allocated cache capacity, the more system memory is required. However, to maintain the stability of DiskStation, only a quarter of the pre-installed system memory can be used for SSD cache. If you want to create an SSD cache of a larger capacity, you might need to upgrade the memory of your DiskStation.

Note:

Every 1 GB of SSD cache requires approximately 416 KB of system memory (expandable memory can be counted). Therefore, if a read-only cache uses RAID 0 and its configuration is 2 x 128 GB SSD, the total cache size will be 256 GB and require at least 104 MB of memory; if a read-write cache uses RAID 1 and its configuration is 2 x 128 GB SSD, the total cache size will be 128 GB and require at least 52 MB of memory. If the memory is insufficient, SSD cache creation will be restricted or the size of SSD cache will be limited.

Note:

Cache Hit Rate

A SSD cache by default only accelerates random I/O and does not accelerate sequential I/O. The Cache Hit Rate of a SSD cache can be calculated according to the following formula:

Managing SSD Cache

To create and mount an SSD cache:

  1. Install the SSDs into your DiskStation. Please see the notes above first.
  2. Go to Storage Manager > SSD Cache. Click Create.
  3. Select the cache mode (only available for DiskStation models that support read-write cache).
  4. Select the volume or LUN on which you want to mount the SSD cache from the drop-down menu.
  5. Select the SSDs from the list. All data on the SSDs will be erased. Please make sure there are no important data on the SSDs.
  6. Click Apply.

To stop caching sequential I/O:

If your target volume or LUN has excellent sequential I/O performance, you can make SSD cache stop caching sequential I/O and access data directly from your drives for higher speed and better performance.

  1. Click Configure to open the configuration window.
  2. Tick Skip sequential I/O.

To remove an SSD cache:

  1. Click Remove to permanently remove this SSD cache from DiskStation.

Note:

To cancel the removal of an SSD cache:

  1. Click Cancel Removal to cancel the removal process.

To repair an SSD cache:

SSD Cache Advisor

This feature allows you to calculate and view the total size and number of recently accessed files on a specific volume. If you have not yet created any SSD cache on the selected volume, this information can be useful when evaluating how much SSD cache capacity is suitable for your system. If you have previously created an SSD cache on the selected volume, the analysis and usage of the cache will also be shown here.

To run SSD Cache Advisor:

  1. Click SSD Cache Advisor to launch the wizard.
  2. Select a volume for calculation, and then click Next.
  3. The wizard will automatically begin totaling the size and number of recently accessed files on the selected volume. Calculation time varies depending on your individual environment.