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Short answer: a NAS needs enough SSD cache to hold the data it repeatedly reads or writes—the workload’s “hot” data—not a fixed percentage of total storage. First confirm that your workload benefits from caching. Small, random, repeated I/O from virtual machines, databases, multi-user file shares and similar services can improve; large sequential transfers and video streaming often gain little. If the NAS is already running, measure its access pattern before buying SSDs. If you are planning a new Synology volume, Synology gives 2.5% of planned volume size as a starting estimate, but that is a vendor-specific planning minimum, not a universal target or performance guarantee.
What SSD cache does in a NAS
An SSD cache is a fast layer in front of slower hard-disk storage. The NAS keeps copies of frequently used blocks on SSDs so repeated requests can be served with lower latency than reading the disks each time. Cache software normally promotes “hot” blocks and eventually evicts data that has not been used recently.
Cache capacity is therefore useful only when the active data set is both smaller than the cache and accessed often enough to be reused. A larger cache can hold more cold data without improving the requests your users actually make. Cache also consumes drive bays, SSD endurance, memory and, for write caching, redundancy resources. Treat it as a workload optimization rather than a general-purpose capacity upgrade.
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Does your NAS workload benefit from cache?
Workloads that commonly fit
SSD cache is most promising for repeated, small-block random I/O. Typical examples include:
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- Several users opening and saving many small files at the same time.
- Virtual-machine disks and other virtualization storage.
- Databases with frequent random reads or writes.
- iSCSI or Fibre Channel storage presented to servers.
- Snapshots, web applications, mail services and backup jobs that revisit the same blocks.
These patterns create cache hits: the same blocks are requested again while they are still in the SSD layer. Latency-sensitive applications may feel more responsive even when the total amount of data stored on the NAS is very large.
Workloads that often gain little
Large, mostly sequential transfers usually keep the disks busy in a way that a cache cannot materially accelerate. Examples include one-time uploads or downloads of large media files, continuous video playback and sequential backup streams. Synology specifically cautions that gains may be minimal for large-file transfers and video streaming. QNAP’s QTS documentation separates a Random I/O mode, aimed at virtualization and databases, from an All I/O mode that also covers video streaming and large-file access; those modes are software-specific and should not be treated as proof that every NAS behaves the same way.
When a cache can be counterproductive
If the frequently accessed data is larger than the maximum supported cache, the hit rate may remain low and the SSDs add complexity without solving the bottleneck. A continuously saturated application can also overwhelm the cache faster than it can serve requests. For ZFS systems, an undersized-memory configuration can make an SSD read cache reduce performance or contribute to instability. Confirm the workload and memory situation before allocating drive bays to cache.
How to size cache from hot data
The most useful sizing rule is simple: make usable cache slightly larger than the aggregate hot-data footprint you expect during normal operation. “Usable” matters because redundancy and cache metadata reduce the capacity exposed to applications.
Synology’s hot-data example
Synology’s DSM 7.1 white paper illustrates the rule with a 2 TB file server containing 100 GB of frequently accessed data. It recommends slightly more than 100 GB of cache. Expanding that cache to 500 GB would not significantly improve performance if the hot data remains 100 GB. This is an illustrative workload example, not a universal benchmark or a promise of a particular speed increase.
Account for usable, not raw, SSD capacity
Two SSDs advertised at a given raw capacity do not necessarily provide that same amount of cache to applications. Read-write cache may reserve space for metadata and redundancy; RAID layouts reduce usable capacity; and the NAS model or operating system can impose a lower maximum. Translate your desired usable cache into the number and size of supported SSDs only after checking the exact model and software version.
Measure an existing NAS before buying
Measurement is more reliable than guessing from total array capacity. Configure the NAS with the services and users it normally handles, then observe a representative busy period that includes peak activity, scheduled jobs and typical file access.
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Synology SSD Cache Advisor
For an existing Synology volume, Synology recommends SSD Cache Advisor. Enable the services expected in normal use, select the relevant volume and start the advisor from the storage-management interface. The initial analysis takes at least seven days and ends automatically after 30 days. It uses observed usage patterns to recommend a cache size, subject to the model’s cache and memory limits.
Do not run the advisor during an unusually quiet week and assume its result represents a busy production period. Include normal virtual machines, database activity, multi-user file access and scheduled backups. The recommendation is bounded by what the hardware supports, so a workload can still need more cache than the model permits.
If your platform has no sizing advisor
Use workload statistics from the NAS or the application: repeated random-read and random-write rates, block sizes, latency, cache-hit information and the amount of data accessed during peak windows. Look for the smallest hot set that covers the recurring activity rather than the total data stored. If those statistics are unavailable, start conservatively, monitor latency and hit behavior, and expand only when measurements show that useful hot blocks are being evicted.
Planning a new volume: what the 2.5% figure means
When a Synology volume has not yet been created, Synology suggests starting with at least 2.5% of the planned volume size. For example, a planned 2 TB volume would produce a 50 GB arithmetic starting point. The figure is a Synology planning estimate for an uncreated volume, not an industry rule, measured performance result or guarantee that 50 GB covers a particular application’s hot data.
Once the volume is operating, replace the estimate with observed usage. A database or virtual-machine workload may need a cache close to its active working set, while a media library may gain little even if 2.5% would be easy to allocate. If the estimate exceeds the model’s supported cache or memory allowance, the hardware limit takes precedence.
Read-only versus read-write cache
Read-only cache
A read-only cache stores copies of data that can be fetched again from the main volume. Losing the cache drive should not destroy the authoritative data, although the NAS may need to rebuild the cache and performance can fall during that process. Synology’s DSM help describes read-only cache as using at least one SSD.
Read-write cache
A write-capable cache acknowledges writes before they reach the hard-disk volume, then destages them later. That can reduce write latency, but an SSD failure or power loss can jeopardize data that has not yet been synchronized. Synology’s DSM documentation describes read-write cache as requiring at least two SSDs with redundancy. Follow the vendor’s supported RAID layout and power-loss guidance rather than assuming that any pair of SSDs is safe.
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Before removing a write-cache SSD, use Storage Manager’s cache-removal procedure. Synology warns that unsynchronized data may remain on the cache; physically pulling a drive first can cause data loss or an unclean volume state.
Memory and platform limits
Synology DSM guidance
For DSM 7 guidance, Synology states that each 1 GB of SSD cache needs about 400 KB of system memory, with no more than one-fourth of built-in memory allocated for cache support under the documented conditions. DSM 6.2 and earlier uses a different calculation. These values are version- and model-dependent, so verify the support documentation for the NAS you own.
Synology also advises evaluating SSD endurance, sustained performance consistency and power-loss protection where applicable. A consumer SSD that looks fast in a short benchmark may not deliver consistent random I/O under a full cache workload.
QNAP QTS requirements
QNAP’s QTS 5.0.x documentation ties minimum memory to cache size and processor architecture. Its examples list at least 2 GB of memory for a 1 TB cache and at least 4 GB for a 4 TB cache under that QTS version. Those are version-specific minimums, not recommendations that the cache will perform well at those limits. Check the exact QTS release, CPU architecture and model table before planning a large cache.
QTS 4.5.x also documents cache modes and notes safety implications for configurations without disk-failure protection. A mode that permits write acceleration without adequate redundancy can trade latency for a larger failure risk.
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In TrueNAS with ZFS, RAM is the primary read cache, called ARC. An SSD read cache is L2ARC, an additional tier for data that no longer fits in ARC. TrueNAS says L2ARC may help when active data exceeds available RAM but is still small enough that a meaningful portion fits on SSD, particularly for substantial random reads.
L2ARC itself consumes RAM for its index. It does not replace sufficient ARC. TrueNAS advises adding as much RAM as possible before considering L2ARC and warns that inadequate RAM can reduce performance or contribute to instability. For a memory-starved system, upgrading RAM is usually a more appropriate first step than adding cache devices.
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A practical sizing procedure
- Identify the access pattern. List the services using the volume and classify their I/O as random small-block, sequential large-block or mixed.
- Establish a representative observation period. Include normal user activity, virtual machines, database jobs, backups and peak hours.
- Measure the hot set. Use the platform’s advisor or statistics to estimate how much data is repeatedly accessed and how often it is evicted.
- Choose usable cache slightly above that hot set. Leave room for metadata and account for the usable capacity of the supported RAID or redundancy layout.
- Check platform ceilings. Confirm cache limits, required system memory, SSD interface, drive-bay availability and operating-system version.
- Select the cache mode. Use read-only when reads are the objective; use write caching only when the workload needs lower write latency and the supported redundancy and power-loss protections are in place.
- Monitor after deployment. Track latency, cache hits, SSD health, endurance indicators and application behavior. If the hot set is already covered, adding capacity is unlikely to help.
Buying and installation checklist
- Verify that the exact NAS model supports SSD cache and the intended SATA, SAS or NVMe interface.
- Check how many bays or slots the selected cache mode consumes.
- Confirm the model’s maximum cache capacity and memory allowance for the installed operating-system version.
- Use the vendor’s compatibility information and select SSDs with appropriate endurance and sustained random-I/O performance.
- For write caching, verify redundancy requirements and whether power-loss protection is recommended or required.
- Compare the cost and risk of cache with adding RAM or moving a latency-sensitive workload to a dedicated SSD volume.
- Plan maintenance: know how to stop services, flush or safely remove cache and replace a failed SSD.
Common sizing mistakes and fixes
Using a percentage as a universal answer
A percentage of total storage ignores the workload. Treat Synology’s 2.5% figure only as its planning estimate for a new volume, then replace it with measured hot-data information.
Buying the biggest SSDs available
Once the hot set fits, extra capacity mostly stores cold blocks. Spend the budget on endurance, redundancy or memory if those are the actual constraints.
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Ignoring redundancy and usable capacity
Raw SSD labels do not equal usable cache. Recalculate after the supported RAID layout, metadata reservation and model limits are known.
Adding L2ARC before RAM
On ZFS, L2ARC depends on RAM and cannot compensate for an undersized ARC. Increase memory first when the system is memory constrained.
Using write cache without a recovery plan
Write caching introduces a window in which acknowledged data may still be on SSD. Use the supported redundant configuration, power-loss protection where required and the documented safe-removal workflow.
Expecting sequential media work to become dramatically faster
Large sequential transfers and streaming may bypass much of the cache benefit. Measure the workload instead of assuming an SSD layer will remove a disk-throughput limit.
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Choose SSD cache when a shared HDD volume serves a recurring random-I/O workload and the active data set is smaller than a practical cache. Choose more RAM first when the NAS or ZFS pool is memory constrained. Choose a dedicated SSD volume when an application needs consistently low latency, sustained writes or predictable performance for data that should not compete with general-purpose file storage. The correct option depends on the NAS model, software version, failure tolerance and measured workload—not on cache capacity alone.
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