SATA on KnightLi Blog

Silicon Motion SATA SSD Controller Guide: DRAM Cache vs DRAM-less XT Series

Sat, 30 May 2026 11:17:22 +0800

Silicon Motion’s SATA SSD controllers can be roughly split into two lines: standard versions with independent DRAM cache, which focus on performance and stability, and DRAM-less XT models, which focus on cost and entry-level markets.

Because the SATA 3.0 interface itself is limited to 6Gbps, real sequential read/write throughput usually tops out around 550MB/s to 560MB/s. Many controllers can look similar in empty-drive benchmarks. The real difference shows up in sustained writes, random I/O, near-full dirty-drive behavior, NAND quality, and firmware tuning.

Two Product Lines

Type	Representative Models	External DRAM	Main Positioning	Strengths	Weaknesses
Standard / cached	SM2246EN, SM2256, SM2258, SM2259	Yes	Mid-to-high-end SATA SSDs, branded drives, stable system drives	More complete mapping table, better random performance and dirty-drive stability, steadier sustained writes	Higher cost, requires additional DRAM chips
XT series / DRAM-less	SM2246XT, SM2254XT, SM2258XT, SM2259XT, SM2259XT2	No	Entry-level SSDs, old-PC upgrades, external enclosures, white-label drives	Low cost, mature solutions, suitable for light workloads	More likely to slow down during large sustained writes or near-full use

Cached SATA Controllers

Cached versions support external DRAM cache, such as LPDDR3, DDR3, or DDR4. They can keep a more complete mapping table, so they are usually steadier than DRAM-less solutions for long-term system-drive use, large sustained writes, and near-full drives.

Model	Technology Stage	Typical NAND	Main Features	Representative Use / Reputation
SM2246EN	Classic MLC era	2D MLC / early TLC	Single-core architecture, 4 channels, low power, low heat, high efficiency	One of Silicon Motion’s famous consumer SSD controllers; often called a “white-label magic chip” in used-drive, white-label, and SSD DIY circles
SM2256 / SM2256K	TLC transition	2D TLC	Built for TLC NAND, with NANDXtend and LDPC error correction	Helped TLC SSDs move from cheap experiments toward large-scale consumer adoption
SM2258	Mature 3D TLC era	3D TLC	4 channels, external cache support, strong error correction, balanced overall behavior	Common in mid-to-high-end SATA drives such as early Crucial MX500 versions
SM2259	Later / modern SATA	High-layer-count 3D TLC / QLC	An improved SM2258-class design with lower power and better support for newer high-layer-count NAND	Seen in later MX500 versions and some Kingston SATA products

DRAM-less XT Series

The XT series removes the external DRAM cache chip and stores the mapping table in controller SRAM or a reserved NAND area. Its goal is not to chase high-end performance, but to reduce total drive cost.

Model	Relationship / Stage	Typical NAND	Common Uses	Notes
SM2246XT	DRAM-less version of SM2246EN	2D MLC / early TLC	Low-cost SSDs, industrial control devices, old-PC upgrade drives	Although DRAM-less, the MLC-era NAND base often gave it much better experience than HDDs
SM2254XT	DRAM-less model from the TLC transition period	2D TLC	Some OEM or custom solutions	Relatively uncommon; judge finished drives by NAND and firmware, not only controller model
SM2258XT	Common DRAM-less controller in the 3D TLC era	3D TLC	Entry-level branded drives, white-label SSDs, flashing/open-card drives	Low cost and good compatibility, but heavier loads and dirty-drive use expose slowdowns
SM2259XT	DRAM-less line corresponding to SM2259	High-layer-count 3D TLC / QLC	Entry SATA SSDs, external enclosures, low-cost upgrade drives	Empty-drive sequential speed may look fine, but long-term system-drive stability is below cached versions
SM2259XT2	Further simplified SM2259XT variant	High-layer-count 3D TLC / QLC	Low-cost designs paired with a single high-capacity NAND package	May reduce channel count, lowering cost but also limiting sustained performance

Generation Mapping

Technology Generation	Cached Version	DRAM-less Version	Typical NAND	Keywords
Classic / early	SM2246EN	SM2246XT	2D MLC / early TLC	Low power, stable, common in DIY circles
TLC transition	SM2256 / SM2256K	SM2254XT / early 2256XT	2D TLC	NANDXtend, LDPC, TLC adoption
Mature 3D NAND	SM2258	SM2258XT	3D TLC	SATA performance approaches interface limit
Later / modern	SM2259	SM2259XT / SM2259XT2	High-layer-count 3D TLC / QLC	Power optimization, high-layer NAND support, low-cost designs

Buying Guide

Use Case	Better Choice	Reason
Light old-PC upgrade	DRAM-less drives such as SM2258XT / SM2259XT can be acceptable	Low cost; usually enough for launching apps, office work, and light system use
Long-term system drive	Prefer cached SM2258 / SM2259 solutions	More stable during random I/O, dirty-drive use, and near-full operation
Frequent large-file writes	Prefer cached versions	After SLC Cache is exhausted, cached solutions usually slow down more gently
External enclosure / temporary data drive	XT series is acceptable	Cost comes first, and workloads are usually simpler than system-drive workloads
Cheap white-label or open-card drive	Check NAND and firmware, not just the controller	The same controller can behave very differently with different NAND
Near-full use	Prefer cached versions and leave spare space	DRAM-less drives are more likely to show obvious fluctuations when dirty

Why Sequential Benchmarks Are Not Enough

Metric	Empty-Drive Sequential Read/Write	Key Behavior in Real Use
Limiting factor	SATA 3.0 physical bandwidth	Controller, cache, NAND, firmware, remaining free space
Common result	Standard and XT versions may both approach 550MB/s	Gaps widen during large writes, random writes, and full-drive use
Value for judgment	Shows behavior near the interface limit	Better reflects system-drive and long-term user experience

So SM2259 and SM2259XT may look close in empty-drive sequential benchmarks. But during long-term system-drive use, large writes, SLC Cache exhaustion, or low free space, cached versions are usually more stable.

Quick Conclusion

Need	Recommendation
Stable system drive	Prefer cached standard-version solutions such as SM2258 and SM2259
Low-cost old-PC upgrade	DRAM-less solutions such as SM2258XT and SM2259XT are worth considering
White-label / low-end / open-card SSD	Do not only check controller model; also check NAND, channel count, and firmware
Judge SATA SSD quality	Do not only look at sequential benchmarks; check dirty-drive behavior, random performance, and sustained writes

In short: Silicon Motion’s standard SATA controllers are better for stability and long-term experience, while the XT series is better for low cost and light workloads. Now that SATA is already close to its interface ceiling, cache design, NAND pairing, and firmware tuning matter more than a simple 550MB/s benchmark result.

TerraMaster F2-221 NAS Backplane Pinout Notes

Mon, 04 May 2026 06:02:56 +0800

This note documents the non-standard backplane connector pinout of the TerraMaster F2-221 NAS. The connector looks close to a PCIe edge connector, but it is not a standard PCIe slot. It is a custom TerraMaster backplane interface.

The connector carries SATA, power, reset, and PCIe signals at the same time. Once PCIe1 x1 is confirmed usable, a custom backplane can expose an M.2 M-key slot and use an NVMe SSD as an internal system drive.

The same idea also applies to the TerraMaster F2-220. Although the F2-220 and F2-221 use different platforms, a fnNAS forum test shows that F3 Backplane V1.1 can detect NVMe on the F2-220, and the NVMe drive is visible inside the OS installer. The extra work is that the old BIOS may not support booting from NVMe.

Summary

The F2-221 backplane connector contains:

Signals for two native SATA ports
12V, 5V, 3.3V, and GND
SATA drive power-control related signals
PERST#
At least one usable PCIe Gen2 x1 signal group
Partial clues for a second PCIe signal group, but not fully verified

PCIe1 can be used to expose an M.2 M-key NVMe slot. In testing, the NVMe drive runs at PCIe Gen2 x1, and the BIOS can detect and boot from it.

F2-220 testing points in the same direction: the hardware can detect NVMe, but the BIOS boot stage may require injecting an NVMe module, and the boot entry may appear as PATA.

Backplane Connector Pinout

The connector has B/A sides. ? means unknown or unconnected, and NC means not connected.

Pin	B side	A side
1	12V	?
2	12V	12V
3	12V	12V
4	GND	GND
5	SATA1 A+	SATA1 B+
6	SATA1 A-	SATA1 B-
7	GND	NC
8	5V	5V
9	5V	5V
10	?	5V
11	?	?
12	3.3V	GND
13	GND	3.3V
14	SATA2 A+	3.3V
15	SATA2 A-	GND
16	GND	SATA2 B+
17	PERST#	SATA2 B-
18	GND	GND
19	PCIe1 TX+	NC
20	PCIe1 TX-	GND
21	GND	PCIe1 RX+
22	GND	PCIe1 RX-
23	PCIe1 REFCLK+	GND
24	PCIe1 REFCLK-	GND
25	GND	PCIe2 RX+
26	GND	PCIe2 RX-
27	PCIe2 TX+	GND
28	PCIe2 TX-	GND
29	GND	PCIe2 REFCLK+
30	?	PCIe2 REFCLK-
31	?	GND
32	GND	?

PCIe1 has higher practical reference value. PCIe2 is not fully verified and should only be treated as a clue, not a reliable design basis.

Signal Source Reasoning

The stock F2-221 dual-bay backplane does not include a PCIe-to-SATA controller. SATA signals go directly from the motherboard connector into the backplane. The extra PCIe signals are mainly inferred from other multi-bay models in the same product family.

The TerraMaster F5-422 backplane uses two ASMedia ASM1061 chips. ASM1061 is a PCIe Gen2 x1 to dual-SATA controller. Combined with the Intel J3355 having 2 SATA ports and 6 PCIe Gen2 lanes, this suggests that multi-bay models expand SATA ports through PCIe.

Therefore, it is reasonable for the F2-221 motherboard connector to retain PCIe signals. The vendor likely reuses motherboard designs across models with different bay counts and changes functionality through the backplane.

PCIe Differential Pair Identification

PCIe differential pairs often go into inner layers after vias, so photos alone cannot trace the complete routing. One useful rule is that, in traditional PCIe designs, TX differential pairs usually have AC coupling capacitors.

The direction must be viewed in reverse:

TX from the ASM1061 controller’s perspective corresponds to RX on the CPU or motherboard side.
RX from the ASM1061 controller’s perspective corresponds to TX on the CPU or motherboard side.
REFCLK needs to be judged together with neighboring differential pairs and routing location.

This kind of pinout is better treated as hardware reverse-engineering material, not as an official specification.

Validation

F3 Backplane designs based on this pinout have completed the following validation:

The original two SATA drive bays remain usable.
PCIe1 can be routed to an M.2 M-key slot.
The NVMe SSD can be detected by BIOS.
The NAS can boot directly from the NVMe SSD.
btrfs scrub found no disk errors.
The system ran from the NVMe SSD for weeks without obvious issues.

The test NVMe SSD was a Patriot P300 128GB. hdparm result:

1
2
3

/dev/nvme0n1:
 Timing cached reads:   4554 MB in  2.00 seconds = 2279.68 MB/sec
 Timing buffered disk reads: 1222 MB in  3.00 seconds = 407.22 MB/sec

This speed matches the PCIe Gen2 x1 limitation. The goal is not to fully utilize NVMe performance, but to replace an external USB SSD with an internal system drive.

Notes

This pinout is useful as a reference for hardware reverse engineering and custom backplanes, but it should not be treated as official documentation.

The connector is not standard PCIe and cannot directly accept generic PCIe devices.
? pins are unverified and should not be connected to critical circuits casually.
PCIe2 is not fully verified and carries higher risk than PCIe1.
CLKREQ is not fully routed like a normal M.2 design, so ASPM may not work.
SATA power includes hot-swap related load switch and slow start logic; do not route only signal lines while ignoring power control.
If reproducing the design, measure your own motherboard and backplane again instead of relying only on photos.

Original project write-up: I made a new backplane for my Terramaster F2-221 NAS
F3 Backplane KiCad project: arnarg/f3_backplane
F3 Backplane pinout CSV: f3_backplane.csv
F2-220 compatibility test: 铁威马F2-220折腾飞牛OS过程