Decide Between an SSD Cache and an SSD Volume
Cache types, array types and drive rules for Synology, QNAP, UGREEN and Asustor
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An SSD cache keeps recently used data on flash in front of a hard drive pool; an SSD volume is flash you store data on directly. Which of the two your M.2 slots can do is set by the maker. QNAP lists three cache types, read-only, read-write and write-only. UGREEN's slots take a storage volume or a read-only cache. Asustor lists full array types for an M.2 volume. On Synology's 2025 models, from DSM 7.3, only the 7 Synology drives on its list can create a cache or a pool at all.
What each maker's documents say
| Maker | Cache | Volume on M.2 | Drive rule |
|---|---|---|---|
| Synology, 2025 models | Allowed with a listed drive | Storage pool, with a listed drive | Only the 7 Synology SNV drives on its list can create either; an unlisted M.2 drive cannot |
| Synology DS723+ | Read and write cache, on M.2 NVMe or 2.5-inch SATA SSD | SSD storage pool | Synology SNV3400 and SNV3410 drives enable caching or a pool |
| QNAP TS-262A, TS-462A, TS-473A | Read-only, read-write and write-only; cache arrays in RAID 0, 1, 5, 6 or 10 | Not stated on the pages read | M.2 drives are not hot swappable |
| QNAP TS-473A under QuTS hero | Read cache, a read and write log cache, and a dedicated ZIL; cache arrays in RAID 0, 1 or 10 | Not stated on the pages read | ZFS on the internal drives |
| UGREEN, models with slots | Read-only cache | Storage volume, 32GB to 8TB per drive | No heatsink; high-power PCIe 4.0 drives not recommended |
| Asustor AS5402T, Lockerstor 4 Gen2+ | Not stated on the pages read | M.2 volume in Single, JBOD, RAID 0, 1, 5, 6 or 10 | M.2 2280 NVMe |
| TerraMaster F4-425 Plus | Not stated on the pages read | Storage pools, including RAID 5 across its three slots | M.2 2280 NVMe, 8TB counted per slot |
Cache or volume
A cache fits when
- The data lives on hard drives and you want repeated reads of the same files to be faster
- Your maker allows only a cache in the slots, as on a read-only-cache design
- You can give a read-write cache two drives, since it holds data that is not yet on the hard drives
A volume fits when
- You want specific data on flash all the time: virtual machines, containers, an app database
- Your maker lists array types for an M.2 volume, so the flash can have its own protection
- You would rather choose what is fast than let a cache decide
Two things the makers' pages make plain
A write cache is not free speed. QNAP lists RAID levels for cache drives, and its ZFS system separates a log cache from a read cache. Data sitting in a write cache is not yet on the pool, so one drive in that role is a single point of failure.
The drive list can decide the question for you. On a Synology 2025 model an M.2 drive that is not one of the 7 on Synology's list can create neither a cache nor a pool, so the choice of drive comes first and the choice of role second.
Where this comes from
The makers' own specification pages, QNAP's software specifications, UGREEN's compatibility notes and Synology's drive policy article, read on October 12, 2026. "Not stated on the pages read" means exactly that: the maker may allow it, but the pages read for this site do not say so.
Checked on October 12, 2026. Sources: Synology Knowledge Center: Drive compatibility policies for systems from 2025 (DSM 7.3 and above); Synology DS723+ datasheet; QNAP TS-473A hardware specifications; Asustor AS5402T hardware specifications; TerraMaster F4-425 Plus specifications; QNAP TS-473A software specifications; UGREEN NAS compatibility list: M.2 SSD notes.
M.2 2280 NVMe drives, by price per TB
250GB to 4TB, the range cache drives are sold in. Check the maker's rule for your model first.
Renewed, refurbished and used listings
Shown for reference. These are not counted in the best price per TB above, and warranty terms differ from new drives.
Frequently asked questions
Which SSD cache types does QNAP support?
QNAP's software specification lists read-only, read-write and write-only SSD cache, with cache arrays in RAID 0, 1, 5, 6 or 10 under QTS.
Can UGREEN M.2 slots be used as a write cache?
UGREEN's M.2 note describes a storage volume and a read-only cache. It does not describe a write cache.
Can any NVMe drive be an SSD cache on a Synology 2025 model?
No. From DSM 7.3, an M.2 drive that is not on Synology's list cannot create a cache or a storage pool on those models. The list holds 7 Synology drives.