SSD Cache

SSD cache can improve the performance of random access by storing frequently accessed data on the SSD cache. SSD cache can be mounted on a volume or iSCSI LUN (Block-Level).

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.

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:

• Read-only cache can consist of 1 to 12 SSDs which can be mounted respectively in a basic or RAID 0 configuration to improve the random read performance of the storage space on which it is mounted.
• Read-write cache can be mounted in a RAID 1 / RAID 5 / RAID 6 configuration, depending on the number of SSDs (up to 12 SSDs), to improve the random read and write performance of the storage space on which it is mounted.

Memory Requirements

Memory requirements depend on the CPU of your DiskStation. To see which CPU is used in your DiskStation, please refer to this article.

For models with Alpine CPUs:

Every 1 GB of SSD cache requires approximately 416 KB of system memory (expandable memory can be counted), and the maximum cache size is 930 GB. Therefore, if the read-only cache configuration is 2 x 128 GB SSD, the total cache size will be 256 GB and require at least 104 MB of memory, if the read-write cache configuration is 2 x 128 GB SSD, the total cache size will be 128 GB and require at least 52 MB of memory. If memory is insufficient, SSD cache creation will be restricted or the size of SSD cache will be limited.

For all other models:

SSD cache requires a certain amount of system memory depending on the cache size, so mounting larger SSD cache might require upgrading the memory of the DiskStation. To keep the system stable and fast, only 1/4 of the pre-installed system memory can be used for SSD cache.

Every 1 GB of SSD cache requires approximately 416 KB of system memory (expandable memory can be counted).
Note: This does not apply to read-write SSD caches created on DSM 5.1 or earlier versions. That is, every 1 GB of SSD cache created before DSM 5.2 will still require 4 MB of system memory even when used in the DSM 5.2 environment.

Therefore, if the read-only cache configuration is 2 x 128 GB SSD, the total cache size will be 256 GB and require at least 104 MB of memory; if the read-write cache configuration is 2 x 128 GB SSD, the total cache size will be 128 GB and require at least 52 MB of memory. If memory is insufficient, SSD cache creation will be restricted or the size of SSD cache will be limited.

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:

• Hit Rate of a SSD read-only cache = Accelerated Random Read Counts / Total Random Read Counts * 100%
• Hit Rate of a SSD read-write cache = Accelerated Random Read & Write Counts / Total Random Read & Write Counts * 100%

Managing SSD Cache

To mount 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 models that support read-write cache).
4. Find the drop-down menu and select the volume or iSCSI LUN (Block-Level) on which you wish to mount the SSD cache.
5. Choose the SSDs from the list. Data on the SSDs will be erased, so please make sure no important data will be deleted.
6. Click Apply.

To stop caching sequential I/O:

If your target volume or iSCSI LUN (Block-Level) 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 SSD cache:

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

To cancel removing SSD cache:

1. Click Cancel Removal to cancel the cache removing action.

To repair SSD cache:

• For read-write cache: If one of the SSDs has crashed, please follow the steps below to repair it.
  1. Replace the failed SSD with a new one. (The new SSD must be the same model and same size as the old one).
  2. Click the Repair button.
  3. Select the SSD from the list.
  4. Click Apply.
• For read-only cache: If one of the SSDs has crashed, please remove the cache and replace the failed SSD. Then click Create to make a new cache configuration.

SSD Cache Advisor

This function 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 referenced when evaluating how much SSD cache 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 use 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.