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To measure an SSD’s write speed after its fast write region fills, run a long, controlled sequential write and record throughput over time or bytes written. Report the initial burst separately from a stable later interval. A short benchmark score cannot establish post-cache performance.
What this benchmark measures—and what it does not
A long sequential write trace shows how quickly a drive handles a sustained stream of sequential data under the conditions you tested. It can reveal an initial fast burst, a drop, and any later plateau or recurring dips. That is different from random-write IOPS, which measures small, scattered operations rather than large-copy throughput. The fio documentation describes SPRandom as a preconditioning method for steady-state random-write workloads; it is not a substitute for a sequential post-cache test. See fio documentation.
There is no broadly applicable cache-size or post-cache speed figure for SSDs. Behavior varies with drive model and capacity, free space, prior writes, temperature, and background drive activity. SNIA also notes that fresh-out-of-box peak behavior and preconditioning matter when measuring steady state. Its background discussion appears in the October 2020 Introduction to SNIA Persistent Memory Performance Test Specification, v1.0; that paper is not the current SSD performance specification.
Prepare a safe, repeatable test
Choose the test path and protect your data
- Back up important data and verify the target drive before starting. A raw-device write test can overwrite its target.
- A file-based test avoids selecting a raw block device, but its result also reflects the filesystem and available free space. Record which method you used.
- Do not leave a long test unattended if temperature or write limits could become a concern.
Record the drive and system state
Before each run, note the exact drive model and capacity, firmware, interface and negotiated link, host, operating system, test program and version, cooling, and temperature. Record whether the drive is fresh, partly filled, or has already received substantial writes. SNIA’s SSD guidance emphasizes standardizing test hardware, software, settings, methodology, and procedure when results are intended to be repeatable or comparable. See SNIA’s Solid State Storage resource and its Understanding SSD Performance Project.
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Set up a sequential write workload
Choose settings that reflect the large-copy workload you want to understand, then keep them fixed between runs. With fio, disclose the block size (bs), queue depth, job count, direct-I/O or buffered-I/O setting, and whether writes are flushed or synchronized. The fio documentation defines bs as the I/O unit size; settings affect what the test represents, so a result without them is hard to interpret.
There is no single consumer fio command that is right for every drive and operating system. Select the correct file or device target and configure a sequential write workload long enough to show the burst-to-sustained transition. If using a raw device, verify the target carefully because writes may destroy data. For formal comparison, consult the applicable SNIA specification rather than assuming a desktop fio run follows it.
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Run long enough to see the transition and later behavior
Write enough data to pass the drive’s current fast write region, then continue long enough to see whether throughput settles, oscillates, or recovers. No universal number of gigabytes or minutes guarantees that the cache has filled: cache behavior can depend on model, capacity, free space, prior writes, temperature, and background folding or cleanup. Do not treat a guessed cache size as proof that the fast region is exhausted; examine the trace.
Preserve throughput in short, fixed intervals or equivalent telemetry, along with cumulative bytes written, elapsed time, temperature, and any pauses. Report the initial burst, where or across what range throughput fell, and the median or average for a clearly identified stable post-transition window. If the trace has no distinct transition or stable interval, say so and report the trace rather than presenting a post-cache number.
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Repeat runs and compare drives fairly
Repeat from a documented drive state. If the comparison is meant to represent cooled drives, allow the same idle or cooling period between runs. If the real workload writes continuously without a break, model that instead. Keep fullness consistent: a nearly empty drive and a heavily occupied one may not behave alike.
For each drive, compare the transition, sequential throughput over the same post-transition window, variation or periodic dips, temperature, and test path. Also disclose the fill state and host link. SNIA cautions that workload content and intensity affect measured bandwidth and response time, so a sequential-copy result does not settle questions about random performance or endurance.
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Interpret a slowdown without over-attributing it
A sudden or gradual drop after fast writes is consistent with the drive moving beyond a limited high-speed write cache, but the trace alone may not identify the controller’s internal cache policy. Thermal throttling, host or interface limits, filesystem behavior, background garbage collection, drive fullness, and write-durability settings can also affect throughput. Check temperature and host link limits, then repeat under controlled conditions before attributing a drop to one cause.
SNIA says, “SSDs behave very differently than HDDs, and standard HDD benchmarks will not provide a true picture of SSD performance.” Its formal Performance Test Specifications are designed for standardized comparison of client and enterprise SSDs and recommend a Reference Test Platform. A practical desktop test can answer an individual user’s question, but it should not be described as SNIA-compliant unless the applicable specification and setup were followed. See SNIA’s SSD performance project.
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