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For an existing Synology volume, use SSD Cache Advisor rather than guessing. Its initial analysis takes at least seven days and ends automatically after 30 days. For a Synology volume that has not been created yet, Synology gives “at least 2.5%” of planned volume size as a starting estimate. That is a vendor-specific planning figure, not a universal target or performance guarantee.
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What SSD cache actually does
An SSD cache is a faster layer in front of slower hard-disk storage. Frequently requested blocks can be served from SSD instead of being read from the disk array. With write caching, incoming data may be staged on SSD before it is committed to the storage pool. The benefit depends on cache hits: data must be requested repeatedly, and the access pattern must match what the NAS’s cache software can accelerate.
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Repair common Windows errors and clear accumulated junk for a smoother, more stable PC - no reinstall needed.Free scan · no reinstallCache capacity is therefore a workload question. A 2 TB NAS with 100 GB of data that users repeatedly open may benefit from roughly 100 GB-plus of usable cache. A 100 TB archive whose files are each read once may gain little from even a very large cache. Cache also has limits imposed by the NAS model, operating-system version, available memory, SSD interface and drive bays.
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No independent, broadly applicable statistic establishes a guaranteed percentage speed improvement from NAS SSD caching. Results vary with the application, block size, queue depth, number of users, disk layout and cache mode.
First decide whether your workload is cache-friendly
Measure the access pattern before buying SSDs. Caching is most promising when the same small blocks are read or written repeatedly, especially when the active data set is larger than the NAS’s RAM but small enough for a meaningful portion to fit on SSD.
| Workload pattern | Likely cache value | Why |
|---|---|---|
| Many users opening and saving small files | Often worthwhile | Repeated, small random I/O can produce cache hits. |
| Virtual machines | Often worthwhile | VM boot disks and virtualized workloads commonly generate random reads and writes. |
| Databases | Often worthwhile | Indexes and frequently used records may be accessed repeatedly. |
| iSCSI or Fibre Channel storage | Workload-dependent | Block traffic can be random or sequential; measure the actual clients. |
| Snapshots, web services, backup tasks or mail services | Workload-dependent | Repeated metadata and small-block operations may benefit, but a continuously saturated application can exceed cache capacity. |
| Large-file copying, media libraries and video streaming | Often limited | Mostly sequential transfers can bypass much of the cache benefit. |
Synology specifically cautions against relying on cache when the frequently accessed data exceeds the maximum supported cache size or when an application is continuously under high load. QNAP’s QTS documentation distinguishes Random I/O mode, with virtualization and databases as examples, from All I/O mode, which can include video streaming and large-file access. Those modes are vendor-specific; one NAS’s behavior should not be assumed to apply to another.
How to size cache on an NAS you already use
Use observed demand instead of total pool size
If the NAS is in production, let it observe normal activity. Enable the file shares, virtual machines, databases, backup jobs and other services that will actually run, then collect data during a representative busy period. A week that includes only a quiet weekend will understate demand; a one-time migration can overstate it.
Synology: run SSD Cache Advisor
On Synology DSM, open the storage-management tools for the relevant volume and start SSD Cache Advisor. Synology says the initial analysis requires at least seven days and ends automatically after 30 days. The advisor uses observed usage patterns to recommend a cache size. Treat that result as a recommendation bounded by the model’s supported capacity and memory, not as a promise of a particular speedup.
- Finish configuring the volume and enable the services used in normal operation.
- Run the NAS through a representative workload for at least the advisor’s seven-day initial period.
- Review the suggested size and compare it with the model’s maximum cache, SSD-bay and memory limits.
- Translate the required usable cache into raw SSD capacity after accounting for redundancy.
- Recheck performance and cache-hit behavior after deployment rather than assuming a larger cache will continue to help.
Synology’s white paper recommends a cache slightly larger than the aggregate hot data. Its example uses a 2 TB file server with 100 GB of frequently accessed data: slightly more than 100 GB of cache is suggested, while expanding the cache to 500 GB would not significantly improve performance if the hot set remains 100 GB. This is an illustration of the sizing principle, not a universal benchmark.
Estimating cache for a new volume
Use the 2.5% figure only as a planning baseline
When a Synology volume has not yet been created, Synology’s September 2024 guidance says to estimate at least 2.5% of the planned volume size. For a planned 8 TB volume, that arithmetic gives 200 GB as an initial estimate. It does not mean every 8 TB NAS needs 200 GB, nor does it account for the actual hot-data footprint, cache mode, RAID overhead or the model’s limits.
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| Planned volume size | 2.5% planning estimate | Qualification |
|---|---|---|
| 2 TB | 50 GB | Synology minimum planning estimate for a volume not yet created. |
| 8 TB | 200 GB | Starting point only; measure after deployment. |
| 20 TB | 500 GB | Not a guarantee that 500 GB of cache will be useful. |
For other NAS platforms, do not transplant Synology’s percentage. Start with an estimate of the active data set, then check the platform’s documented cache limits and memory requirements.
Size for the hot set, with a small margin
List the data that is repeatedly accessed during normal operation: VM disks, database indexes and working tables, project shares, metadata-heavy applications and frequently used documents. Estimate the aggregate size of that set, then choose a usable cache slightly above it if the platform supports that configuration. If the hot set is unknown, run the vendor’s monitoring tool or collect representative I/O statistics before committing to drives.
Convert usable cache into real SSD capacity
The cache size exposed to applications is not always the raw capacity printed on an SSD. Read-write cache commonly uses mirrored or otherwise redundant SSDs, so two drives may provide less usable space than their combined raw capacity. Filesystem metadata, cache-reservation rules and model limits can reduce it further.
For example, a requirement for 200 GB of usable write cache may call for two larger SSDs in a mirrored arrangement rather than one 200 GB drive. The exact layout depends on the NAS vendor and model. Check whether the system reports cache capacity before or after RAID overhead, and verify the minimum and maximum drive sizes it accepts.
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Memory and platform limits change the answer
Synology DSM
Synology’s DSM 7 guidance estimates about 400 KB of system memory for each 1 GB of SSD cache, with no more than one-quarter of built-in memory allocated to cache support under the documented conditions. DSM 6.2 and earlier use a different calculation. These figures are specific to the stated DSM guidance and model conditions, not a universal NAS rule.
Synology also differentiates cache modes: read-only cache can use at least one SSD, while read-write cache requires at least two SSDs with redundancy in the documented setup. Confirm the exact NAS model, DSM version and supported SSD interface before buying drives. Synology advises considering endurance, sustained performance consistency and power-loss protection when selecting SSDs.
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QNAP QTS
QNAP’s QTS 5.0.x documentation ties minimum memory to cache size and processor architecture. It lists, under the stated version, at least 2 GB of memory for a 1 TB cache and at least 4 GB for a 4 TB cache. These are minimum requirements, not recommendations that those cache sizes will improve a particular workload.
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QTS 4.5 documentation describes Random I/O and All I/O modes and notes safety implications for configurations without disk-failure protection. Because requirements differ by QTS release, CPU architecture and model, use the documentation for the exact system rather than a generic calculator.
TrueNAS and ZFS
TrueNAS uses RAM as the ZFS Adaptive Replacement Cache (ARC) and can add SSD read cache as L2ARC. L2ARC may help when active data exceeds RAM but is still small enough that a useful portion fits on SSD, particularly for substantial random reads.
L2ARC itself consumes RAM. TrueNAS advises adding as much RAM as possible before considering L2ARC and warns that inadequate RAM can reduce performance or contribute to instability. An L2ARC device is not a substitute for sufficient ARC memory, and adding one to a memory-starved system can make the situation worse.
Read-only versus read-write cache
Read-only cache
Read-only cache stores copies of data that can be recreated from the primary volume. It is generally simpler to deploy because a failed cache device should not contain the only copy of a write that has been acknowledged. Even so, follow the vendor’s supported drive-removal and replacement process.
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Write caching can reduce acknowledgment latency, but it introduces data-integrity and failure considerations. Vendors commonly require multiple SSDs and redundancy for this mode. Use SSDs with the endurance and power-loss protection recommended for the platform, maintain backups, and never treat the cache as an additional backup copy.
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Choose a drive only after confirming the NAS model, operating-system version, supported interface and workload. Compare endurance, sustained random-I/O behavior, thermal characteristics and power-loss protection where the vendor requires or recommends it. A consumer SSD that looks fast in a short benchmark may perform differently after its write cache fills during sustained NAS activity.
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Also compare caching with two alternatives:
- Add RAM first: On systems such as ZFS, more RAM can improve the primary read cache without adding L2ARC overhead.
- Move the hot workload to an SSD volume: If a database or VM workload is consistently latency-sensitive, placing it on an SSD-backed volume may be more predictable than caching a small portion of a hard-disk pool.
- Improve the network path: A 1 GbE link, busy switch or slow client can hide any cache benefit. Cache cannot increase a network link’s line rate.
These alternatives may be more appropriate when the working set is larger than any practical cache or when the application is continuously busy.
Common sizing mistakes and fixes
Buying a percentage without measuring
Symptom: The cache is much larger than the data users repeatedly access, yet application performance is unchanged.
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Fix: Measure the hot set and cache behavior. Reduce the estimate to a usable size that covers the observed working data, subject to the vendor’s minimums.
Caching a sequential workload
Symptom: Large media copies or streaming show little improvement.
Fix: Confirm whether the workload is mostly sequential. Consider faster disks, more network bandwidth or an SSD volume instead of enlarging cache.
Ignoring memory limits
Symptom: The NAS rejects the proposed cache size, becomes memory constrained or performs worse after L2ARC is enabled.
Fix: Check the model-specific memory formula. Add RAM where supported, reduce cache size, or remove L2ARC if the platform’s documentation indicates insufficient memory.
Forgetting redundancy and safe removal
Symptom: A single SSD is selected for a mode that requires redundant devices, or a drive is removed while data is still synchronized.
Fix: Use the supported read-write layout and the NAS’s Storage Manager removal procedure. Verify synchronization before physically replacing a cache drive.
Quick Recap
A practical decision checklist
- Describe the workload: repeated small random I/O, sequential transfers, or a mixture.
- Identify the hot data: VMs, databases, active shares and other repeatedly accessed blocks.
- Measure for a representative period; on Synology, allow at least seven days for SSD Cache Advisor and note that it stops after 30 days.
- Estimate usable cache slightly above the measured hot set, or use Synology’s 2.5% planned-volume figure only as an initial estimate for a new Synology volume.
- Check model, OS version, RAM allowance, cache-mode requirements, SSD interface and drive-bay limits.
- Account for RAID or mirroring overhead when converting usable cache to raw SSD capacity.
- Choose SSDs for endurance and sustained behavior, not headline sequential speed alone.
- After deployment, verify that latency or throughput improves for the workload that motivated the purchase. If it does not, stop enlarging the cache and reassess the workload, RAM, network and storage layout.
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