SSD cache, also known as flash caching, is a cost-effective way to improve the performance of HDD arrays by storing the most frequently accessed data on SSDs.

Read the information below to explore whether you will benefit from adding an SSD cache and which setup is right for your use case.

Recommended applications

Unsuitable applications

Read-only and read-write cache

Memory requirements

Selecting suitable SSDs

Further reading

First assess whether SSD cache will provide a significant boost to your application, then decide whether to create a read-write or read-only cache and finally select appropriate drives.

Recommended applications

SSD cache enhances performance where input-output (I/O) operations require frequent access to randomly placed small blocks of data.

SSD cache is likely to boost performance if you use your Synology NAS for these applications:

• File server (the more concurrent users are connected and small files¹ are accessed, the bigger the performance gains)
• iSCSI and Fibre Channel storage
• Synology Virtual Machine Manager
• Database storage
• Snapshot
• Webserver
• Regular backup tasks with Synology Active Backup for Business
• Mail services

Using SSD cache is not recommended if the amount of frequently accessed data on your Synology NAS exceeds the maximum size of SSD cache or if an application is always under a high load. Cache flushing is resource-intensive and can impact performance if there are no off-peak hours. We recommend storing frequently accessed data and running high-load applications on all-SSD volumes to speed up operations.

Unsuitable applications

SSD cache will not improve performance in scenarios involving sequential access patterns.

Performance gains from SSD cache are expected to be minimal if you use your Synology NAS for these applications:

• File server used to upload/download/access large files
• File server with mostly sequential access
• Video streaming/playback

Read-only and read-write cache

Synology NAS allows you to choose from two SSD cache types: read-only cache and read-write cache. Both are useful in different applications.

Read-write SSD caches always have redundancy. At least 1 SSD is needed to create a read-only cache, while at least 2 SSDs are required to create a read-write cache.

Memory requirements

SSD caches have the following system memory requirements:

• Depending on your DSM version:
  • DSM 7.0 and above: 400 KB of system memory is required for every GB of SSD cache.
  • DSM 6.2 and below: 416 KB of system memory is required for every GB of SSD cache.
• At most one-fourth of built-in system memory can be used to support SSD cache.
• All add-on system memory be used to support SSD cache.
• You cannot make your SSD cache bigger than system memory allows.

Example: Creating an 800 GB SSD cache on DSM 7.1 requires about 800 x 400 = 320000 KB = 320 MB of system memory.

Selecting suitable SSDs

Three important factors to take into account when selecting SSDs for Synology NAS are their endurance, performance consistency, and the availability of power loss protection.

1. Endurance: The published number of Drive Writes per Day (DWPD) reflects the official maximum number of times a drive can be completely rewritten within the warranty period.
2. Performance consistency: Common benchmarks for SSD performance include sustained 4k random read/write IOPS (Input/Output Operations Per Second) over a set period of time and latency changes over time. Higher IOPS values and stable low latency are important for enterprise environments.
3. Power loss protection: If your SSDs have built-in power loss protection, built-in electronic measures ensure that no data will be lost in case of power loss and that the SSD can remain in operation.

Synology SSDs are built for 24/7 NAS environments and are verified through rigorous validation to be interoperable with Synology systems. They feature end-to-end data protection, integrated power-loss protection, and are available for 2.5" SATA and M.2 22110 interfaces.

Intensive tests, which include I/O stress, power cycling, and temperature trials, ensure that Synology SSDs can provide both the reliability and consistent performance suitable for enterprise environments.

Synology also tests and verifies third-party SSDs. SSD performance varies widely between manufacturers and models. For more detailed warranty and performance information, please consult the SSD manufacturer.