There is no universal SSD size or endurance rating for a NAS. Capacity must hold either the data in an all-flash storage pool or the smaller, repeatedly accessed working set that a cache can accelerate. Endurance must cover the host data written by backups, recordings, virtual machines, databases, snapshots and other jobs over the drive’s service period. Measure those two requirements separately, then verify interface, form factor, firmware and NAS compatibility.
A practical decision is: inventory the data or hot set, add documented growth, account for the usable capacity of your RAID and filesystem, measure representative host writes, project them across the intended ownership period, and compare that result with TBW or DWPD rated under a comparable workload. A drive with impressive headline specifications is still unsuitable if it does not fit the NAS or if the access pattern gains nothing from caching.
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Start by deciding what the SSD does
All-flash storage pool
In an all-flash pool, SSD capacity is the primary storage capacity. Every file, virtual disk or database that belongs on the pool must fit after RAID, pool metadata and filesystem overhead are applied. Start with the dataset that must reside there, add the amount of growth you can substantiate for the planned service interval, and then select a raw capacity that leaves the required usable space after the chosen layout. The supplied sources do not establish a universal percentage of free space or a universal RAID overhead percentage, so calculate those values from your NAS’s actual pool and filesystem implementation rather than applying a generic rule.
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A cache is temporary acceleration space, not a second copy of the NAS dataset. Size it around the frequently reused working set and the cache mode supported by your NAS. Matching cache capacity to the total NAS pool is usually meaningless: a cache only helps when requested blocks are reused while they remain cached. Synology’s cache guidance warns that caching may not be recommended when frequently accessed data exceeds the cache size or when an application is continuously under high load.
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For a cache, first identify which shares, volumes or applications generate repeated reads or writes. Then check whether the NAS can use SATA or NVMe cache in the slot you have, whether it requires one or two SSDs, and whether the cache is read-only or read-write. Do not buy capacity for data that will be streamed once and never revisited.
A capacity calculation you can audit
- Inventory the data. For an all-flash pool, record the current logical data, snapshots and virtual disks that must fit. For a cache, estimate the active working set from file-access reports or application behavior, not from the NAS’s total disk capacity.
- Separate current use from growth. Record a measured growth rate or a documented project requirement and state the period it covers. Do not treat unused bays or a vendor’s maximum volume size as required capacity.
- Apply pool and filesystem effects. Convert raw SSD capacity to the usable capacity of the exact RAID level, pool configuration and filesystem. Include metadata, parity or mirroring loss and any reserved space shown by the NAS management interface.
- Check the physical plan. Confirm that each SSD’s form factor, interface, keying, height and power profile are accepted in the intended bay or M.2 slot. A capacity that cannot be installed or is unsupported is not usable capacity.
- Recheck the access pattern. If the SSD is only a cache, verify that the active set is repeatedly read or written and that the cache implementation can keep it. Sequential streaming or a working set much larger than the cache can produce little improvement.
Keep the calculation in a worksheet with separate columns for logical data, growth, RAID or pool reduction, filesystem overhead and resulting usable capacity. This makes it clear whether a future expansion requires replacing drives, adding a tier or changing the pool.
Workload pattern changes both capacity and usefulness
| NAS workload | Capacity question | Endurance and cache considerations |
|---|---|---|
| Photo libraries, office files and backups | How much data must reside in the pool, and how quickly is it growing? | Measure backup windows and retention jobs. A cache helps only when users repeatedly reopen the same files. |
| Media archives and sequential playback | What is the active library or project size? | Large sequential reads often have little cache reuse. Synology states that large sequential operations such as HD video streaming cannot benefit much from SSD caching because they lack rereading patterns. |
| Video editing and large-media ingest | How large are active projects, proxies and scratch data? | Sustained throughput, interface and write endurance matter alongside capacity. SanDisk lists content creation and video editing as NAS SSD use cases; that is product-category guidance, not independent performance testing. |
| Virtual machines and databases | How many virtual disks and database files must be online? | Measure random and small-block writes, checkpoints and snapshots. Synology identifies OLTP databases and email as examples where non-contiguous I/O can make SSD caching useful. |
| Surveillance and continuous recording | What stream count, bitrate and retention period must be held? | Include the continuous recording stream, indexing and retention deletions in host-write totals. SanDisk lists surveillance footage and continuous recording among NAS SSD use cases. |
Sequential video is not automatically a poor SSD workload; an all-flash pool may still be chosen for latency or throughput. The narrower point is that a cache cannot accelerate data that is read once and displaced before it is read again.
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Measure writes before choosing endurance
Endurance answers a different question from capacity: how much data can the SSD accept over time? Use host-write counters from the NAS, SSD SMART data or the storage-management system. Capture a representative busy period that includes ordinary user activity plus backups, recording, virtual-machine traffic, database maintenance, snapshots, replication and scheduled scrubs. A quiet week can dramatically understate a production workload.
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- Record the host-written total at the beginning and end of the measurement period.
- Divide the difference by the number of days to obtain average daily host writes.
- Repeat during a known peak period and retain both average and peak figures.
- Multiply the representative daily figure by the number of days in the intended service interval. If you are comparing with a warranty-based rating, use its stated years.
- Compare the projected writes with TBW or with the daily writes implied by DWPD, then investigate any margin that is too small for workload variation.
Write totals should be in consistent decimal units. A terabyte in the endurance calculation should be treated consistently with the drive specification; do not mix decimal TB with binary TiB without labeling the conversion.
TBW, DWPD and the formulas
TBW
Terabytes written (TBW) is the total host data written over the endurance rating. It is not a promise that every workload will produce the same NAND wear for the same host-write count. The rating is established under a defined test workload and warranty or service period.
DWPD
Drive writes per day (DWPD) expresses how many complete drive-capacity writes per day are supported over the rating period. SNIA gives the simplified relationship:
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Rearranged, estimated rated daily host writes are:
daily writes ≈ DWPD × drive capacity in TB
If only TBW is available, calculate:
rated daily writes ≈ TBW / (365 × warranty years)
These are comparison formulas, not a guarantee for your NAS. Random and small-block writes, the amount of free space and overprovisioning, TRIM behavior and the SSD’s internal write amplification can change the relationship between host writes and NAND writes. Compare products using the workload basis on which their ratings were produced.
Workload basis matters
Synology says its SSD TBW specifications are calculated using the JESD219A enterprise workload. A NAS dominated by large sequential backups, a database with random overwrites and a surveillance recorder can therefore produce different wear for the same host-write total. Kingston’s technical explanation likewise frames DWPD as a conversion from TBW that depends on drive capacity and warranty duration.
Worked examples using published specifications
The following arithmetic illustrates how to read a rating; it does not establish a minimum NAS requirement.
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Converting a 400 GB, 491 TBW rating
Synology lists the SNV3410-400G at 400 GB, 491 TBW and a five-year limited warranty on its current product page accessed in 2026. Treating 400 GB as 0.4 TB, the simplified conversion is approximately 491 ÷ (365 × 5 × 0.4) = 0.67 DWPD, or about 491 ÷ 1,825 = 0.269 TB (269 GB) of rated host writes per day. Your NAS workload may not match the JESD219A basis, so this result is an estimate for comparison, not a daily allowance to exceed casually.
Comparing higher-TBW variants
Synology’s current SNV5420 page lists 400 GB, 800 GB and 1,600 GB variants with more than 700 TBW, 1,400 TBW and 2,900 TBW respectively. The product page figures are vendor specifications accessed in 2026; a warranty duration for these particular entries is not stated here, so a DWPD conversion would require that missing duration. Compare the capacities and TBW values only as stated, and verify the exact SKU before purchase.
Reading a broad product family range
SanDisk lists NAS 600 and NAS 800 families spanning 500 GB to 4 TB in SATA models and 960 GB to 7.68 TB in NVMe models, with up to 2,500 TBW for SATA and 14 PBW for NVMe. Both families list a five-year limited warranty. “Up to” applies to the highest-rated model, not every capacity; check the exact model’s TBW, interface and regional warranty terms.
Compare drives on more than the headline number
| Comparison axis | What to verify | Why it changes the decision |
|---|---|---|
| Usable capacity | Raw size, RAID or pool layout, filesystem overhead and growth | Raw terabytes do not equal the data space available to shares or virtual disks. |
| Endurance | TBW or DWPD, warranty duration and rating workload | A larger TBW is not directly comparable if the workload basis or warranty period differs. |
| Interface and form factor | SATA or NVMe, bay type, M.2 key, physical height and power limits | The NAS may reject an otherwise suitable drive or lack the expected throughput. |
| Firmware and platform support | NAS vendor compatibility list, minimum firmware and supported features | Unsupported firmware can cause warnings, missing health data or failed pool creation. |
| Cache mode | Read-only versus read-write and required number of SSDs | Read-write caching has different failure and redundancy requirements. |
| Warranty and region | Exact SKU, warranty years, TBW conditions and local terms | Published family ranges and marketing names may not apply to the model sold in your region. |
Make SSD caching safe enough for the workload
Read-only caching stores frequently accessed data for random reads. Losing a cache device normally removes acceleration rather than the source data, but the NAS must be able to rebuild or invalidate the cache correctly.
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Read-write caching is more consequential because writes are acknowledged through SSD storage. Synology’s guidance says at least two SSDs in RAID 1 are needed to tolerate one SSD failure in a read-write cache. Simultaneous wear-out or failures beyond the RAID fault tolerance can still risk data loss. RAID is not a backup: maintain an independent backup of the underlying NAS data and test restoration.
Do not enable read-write caching merely because SSDs are available. Confirm that the NAS supports the mode for the volume, that the SSDs are approved for it, and that the measured write stream fits the endurance plan. If the workload is mostly one-pass sequential video, a cache may consume endurance without delivering meaningful hits.
Verify compatibility before ordering
- Match SATA SSDs to SATA bays and NVMe SSDs to supported NVMe slots; an adapter does not guarantee NAS support.
- Check the NAS vendor’s compatibility list for the exact capacity and model, not just the interface name.
- Confirm physical dimensions, connector keying, heatsink clearance and thermal limits.
- Read firmware and platform requirements, including whether cache creation requires a particular NAS operating-system release.
- Confirm that health monitoring exposes host writes, temperature and wear indicators for the chosen drive.
- Review the exact warranty, TBW conditions and regional support terms. Family-level “up to” figures are not a specification for every SKU.
What to do when the numbers do not fit
The projected writes approach TBW
Recheck the measurement for snapshots, replication, surveillance retention and backup retries. If the total is real, choose a higher-endurance model, reduce unnecessary rewriting, separate write-heavy applications onto their own pool, or shorten the replacement interval. Do not solve an endurance problem by buying a larger capacity unless the larger model’s TBW actually rises enough for the workload.
Do these 3 things before closing this tab:
1Clear out junk files and repair common Windows errors2Fix the driver behind crashes, sound loss and screen glitches3Repair Windows errors before they cause bigger problemsThe cache is full or hit rates are poor
Measure the frequently reused set and cache-hit behavior. A cache smaller than that set can continually evict useful blocks; a workload with little rereading may not benefit at any size. Consider using SSDs as primary storage for the latency-sensitive dataset instead of expanding an ineffective cache.
The NAS reports incompatibility
Stop before creating a pool or cache. Recheck the exact SKU, interface, firmware and physical slot against the NAS compatibility information. A drive that fits mechanically but is unsupported can still fail qualification, health reporting or firmware updates.
Quick Recap
A concise purchasing checklist
- Role identified: all-flash pool or cache.
- Dataset or hot working set measured, with documented growth.
- Usable capacity calculated after RAID, pool and filesystem effects.
- Representative host writes measured, including peak scheduled jobs.
- TBW or DWPD converted with the correct capacity and warranty units.
- JESD219A or other rating workload compared with the actual I/O pattern.
- Interface, form factor, firmware and exact NAS compatibility confirmed.
- Read-write cache redundancy and independent backup planned.
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