Frequently Asked Question

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). There are two types of SSD 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.
  Note: SSD read-only cache stores copies of data from the volume; thus, no data loss will occur even if the read-only cache has crashed.
  
• 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.

Both types of SSD cache implement an LRU (Least Recently Used) algorithm to swap the data in the cache.

Important:

Before installing any SSDs for usage as cache, please check the following notes:

• 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.
• Please refer to the compatibility list at Synology's website to choose SSD to be used as cache on your Synology NAS.
• 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 Synology NAS. 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)**. 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.
• Please refer to the product specifications of your models on Synology's website for the maximum size of SSD cache allowed on your Synology NAS.
• When a read-write SSD cache is degraded, the caching feature will be disabled, and the system will start writing (i.e. synchronizing) cached data from the SSD to the HDDs. After the data synchronization is completed, no data loss will occur even if the read-write cache has crashed.
• Read-write cache cannot be used together with Time Backup. For example, when you create a read-write SSD cache, you will not be able to run Time Backup. Vice versa, if you are running Time Backup, you will not able to create a read-write SSD cache.

To mount SSD cache:

1. Install the SSDs into your Synology NAS. 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 Synology NAS.
2. For read-write cache, you must remove it through this procedure (even if your Synology NAS is powered off), otherwise volume crash will occur because partial data resides only on the cache.

Note:

• When mounting or removing SSD cache, the system will stop all related services (e.g. shared folders, installed packages, current connections, etc.)
• If a read-write cache on a storage space is not detected when booting up, the system will not mount that storage space. Please find and re-install the SSDs in the Synology NAS and reboot to mount the storage space. If you cannot find the SSDs, you might need to force mount the storage space.

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 caches 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 open 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.

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* SSD cache is only available on certain models.
** 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.