Sequential vs Random Read/Write for Flash — Why Sustained Write Is What Matters
Flash storage behaves very differently for sequential workloads (large contiguous blocks, like video recording) versus random workloads (many small scattered blocks, like app launches or OS tasks). Continuous recording, logging and dashcam use is almost entirely sequential write — and that is exactly what SD Speed Classes and Video Speed Class certify: a minimum sustained write rate. Random I/O performance depends far more on the controller and firmware than on the raw NAND, and is rarely the bottleneck for recording devices. So for a recording or OEM-storage product, specify the sustained write you need (e.g. V30 = 30 MB/s minimum) and verify it on the real device — do not equate a high peak read number, or a USB interface version, with recording reliability.
What "sequential" and "random" actually mean
Sequential access reads or writes a large, contiguous stream — a video file being recorded. Random access performs many small, non-contiguous operations — a database, an app, or OS activity. The two patterns stress different parts of the controller and produce very different numbers on the same card.
Why video recording is a sequential-write problem
A dashcam or IP camera writing H.264/H.265 produces a continuous stream. What matters is the sustained write rate over minutes, not a burst. A card that benchmarks well in a short burst can still drop frames if its sustained write dips during long recording.
What the speed class actually certifies
SD Speed Class (C10), UHS Speed Class (U1/U3) and Video Speed Class (V30/V60/V90) express a minimum sustained write speed, measured sequentially. They answer "can it keep up with continuous recording?", not "how fast are tiny random writes?". That is why they are the right spec for recording devices.
Random I/O is a controller story
Random read/write is dominated by the flash controller, firmware and queue handling. Two cards with identical NAND can differ here. For pure recording or OEM-storage use this is usually secondary — but it matters if the card also hosts an app or operating system, which is what the Application Performance Class (A1/A2) addresses.
How to specify, and what to test
- For recording, specify the required sustained write (e.g. V30 = 30 MB/s minimum) rather than a peak read number.
- Test on the real device with a sustained-write workload, not just a desktop benchmark that measures a burst.
- Do not equate the USB interface version with card write performance — the interface signaling rate is not the drive’s actual throughput.
Not sure which applies to your product?
Send your workload — what writes, how often, at what temperature — and we will propose a configuration with the reasoning. We reply to business enquiries within 1 business day.
Request a quoteFrequently asked questions
My card shows 90 MB/s read but drops frames in the dashcam — why?
Read speed and burst speed are not sustained write speed. Recording needs the minimum sustained write the Speed Class certifies. Check the V/U class and test on-device with a sustained-write tool.
Does a higher capacity card write faster?
Not by capacity alone. Sustained write depends on NAND grade, controller and configuration. WINTEL specifies write speed per project rather than implying a capacity-to-speed rule.
When does random I/O matter for a TF card?
When the card also runs an app or OS (then A1/A2 matters) or handles a logging pattern with many small writes. Pure continuous recording is sequential.
Can WINTEL guarantee a sustained write rate?
State the target sustained write in your RFQ. WINTEL proposes a configuration (NAND grade + controller) to meet it and verifies with write-and-read-back and on-device sustained-write testing before bulk. Any specific class marking requires the part to actually meet it — confirm before claiming a class.
Sources
- SD Association — Capacity (SD/SDHC/SDXC/SDUC)
SDHC spans more than 2GB up to 32GB and uses the FAT32 file system; SDXC spans more than 32GB up to 2TB and uses exFAT. Speed grades cover C2/C4/C6/C10, UHS U1/U3, and Video Speed Class V6/V10/V30/V60/V90.
- USB Implementers Forum (USB-IF) — USB specifications and documents
USB 3.2 Gen 1 (formerly USB 3.0 SuperSpeed) signals at 5 Gbps; Gen 2 at 10 Gbps; USB 4 reaches 40 Gbps and above.
External sources explain industry standards (accessed 2026-10-18); they describe the standard, not WINTEL product parameters.