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Why an SSD cache may not improve NAS performance
A cache is a faster layer in front of the NAS’s main storage, not a general-purpose speed switch. It can serve frequently reused data from SSD instead of fetching it from the underlying drives. If your workload does not reuse data in that way, the NAS still relies largely on its storage array.
Large or sequential transfers are a poor fit
Copying a large file, reading data once, or streaming video typically involves sequential access rather than repeated random access. Synology says SSD cache has limited benefits for large sequential reads and writes, including HD video streaming, and for entirely random data reading patterns. See Synology’s DSM SSD cache help for its workload guidance.
The workload may not revisit the same data
Repeated access to a smaller hot set—such as active virtual-machine data, database files, or metadata-heavy services—is more likely to benefit than a one-pass workload. Synology lists file services, virtualization, databases, snapshots, and web and mail services among potential use cases in its SSD cache considerations.
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The cache may still be warming up
When a requested block is not in cache, the NAS reads it from the underlying storage; the cache may then retain it so a later request can be served from SSD. A new or repurposed cache therefore needs representative use before its statistics reflect the workload. A workload that changes can require another warm-up period. Synology describes this behavior in its considerations article and DSM 7.1 SSD Cache White Paper.
How to troubleshoot an SSD cache that is not helping
- Describe the slow task. Note whether it is small-file browsing, metadata-heavy work, virtual machines, database activity, multiple users, a large transfer, streaming, or surveillance recording. Repeated random access to reused data is a better fit than a one-pass sequential operation.
- Confirm the NAS supports the setup. Check the exact NAS model’s current specifications and compatibility list. Verify SSD interface and form factor, drive health, cache status, and any memory or free-bay requirements. Requirements vary by model, vendor, and software version. For example, QNAP’s QTS 5.2.x SSD cache guide lists a supported NAS model, compatible free SSDs, and suitable installed memory; those details are specific to that guide, not universal NAS requirements.
- Let normal activity populate the cache. Use the NAS as you normally do, then assess performance after a representative period. A benchmark that does not resemble your regular workload may not show whether cache is helping that workload.
- Check cache hit rate in context. Synology defines read-only hit rate as accelerated random reads divided by total random reads; its read-write formula compares accelerated random reads and writes with total random reads and writes. A low rate means more of the relevant requests still go to the underlying storage. It does not, by itself, establish that an SSD is defective: sequential I/O is not accelerated by Synology’s default behavior, and a changing or oversized hot set can also limit hits. See the DSM help page for the definitions.
- Compare the same workload before and after. Measure response time or latency as well as throughput, and check the cache’s own statistics. Compare the same task under similar conditions after warm-up; an unrelated file-copy result is not evidence that cache is working or failing.
- Check for other bottlenecks. A network link, CPU, memory, HDD array, or client device can limit performance even when cache is functioning. The vendor sources do not establish one universal threshold or test for these other limits, so investigate the constraint that matches the observed task.
- Review mode and settings before changing them. Confirm whether the cache is read-only or read-write and which volume or LUN it serves. Synology recommends considering “Skip sequential I/O” when the target volume already has strong sequential performance; follow the procedure for your installed DSM version and model.
Read-only and read-write cache are different choices
Read-only cache is aimed at frequently read data. Read-write cache can apply to read/write patterns, but its supported drive counts, RAID options, and behavior depend on the NAS model and software version. Use the vendor’s model-specific instructions rather than assuming a setup that works on another system will apply to yours. Synology’s cache considerations explain its modes and selection factors.
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Be especially careful when removing a read-write cache. Synology warns that unsynchronized data may still reside on cache, so remove the cache through Storage Manager before taking out the SSDs. Follow the instructions for your DSM version and model.
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What to consider when comparing cache setups
- Workload: Is the activity repeated random access to reused data, or mostly sequential and one-pass?
- Hot-set size and cache capacity: If frequently reused data exceeds the useful cache capacity, hit rate may be limited. Capacity and memory requirements are model-specific.
- SSD characteristics: Check compatibility, interface, form factor, endurance, sustained performance consistency, and power-loss protection where appropriate. Synology discusses these considerations in its SSD cache guidance.
- Measurement: Use the vendor’s cache statistics and a like-for-like workload comparison after warm-up. There is no universal speedup percentage established across NAS models and workloads.
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