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Dividing your required throughput by 625 MBps to get an iSCSI session count or an NVMe/TCP queue count is not an AWS formula, and it did not predict performance in a measured FSx for ONTAP test. In that single-AZ test, the calculation was too low at one connection, roughly right at two, and far too high at sixteen. Treat 625 MBps as a reference point for a single EC2 client, not as a multiplier for planning.
Where the 625 MBps number comes from
The figure traces to AWS’s iSCSI and NVMe/TCP setup procedures for Amazon FSx for NetApp ONTAP. As Yoshiki Fujiwara, an AWS Community Builder, describes those procedures in his DEV Community article (posted September 30, 2026), they state an Amazon EC2 single-client maximum of 5 Gbps, roughly 625 MBps, and then describe optional steps for adding sessions when you want more throughput than that single client reaches.
The step from that context to a formula is the part that does not hold. Fujiwara’s wording on this point is direct: “AWS does not instruct you to divide,” and “No formula is given.” Those lines are his characterization of the linked AWS procedures, not AWS’s own words. In other words, the division is a heuristic someone built on top of a client-side number, and the article’s purpose was to test it.
What the test measured
The test ran on a second-generation FSx for ONTAP SINGLE_AZ_2 deployment, with the following configuration as reported in the article:
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| Item | Reported value |
|---|---|
| AWS Region | ap-northeast-1 |
| File system topology | SINGLE_AZ_2, one 6,144 MBps HA pair |
| SSD capacity | 4,096 GiB |
| Provisioned IOPS | 200,000 |
| Block target | 600 GiB LUN (iSCSI) or namespace (NVMe/TCP) on a 900 GiB volume |
| ONTAP version (iSCSI results) | 9.18.1P5 |
| ONTAP version (NVMe queue results) | Not stated; the queue test ran on a separate deployment whose version was not recorded |
| Client | Amazon EC2 c5n.9xlarge, 50 Gbps guaranteed network |
| Load tool and settings | VDBENCH 5.04.07, direct I/O, unlimited rate (iorate=max), 512 threads, 60-second warmup, 300-second measurement |
| Multi-AZ | Not verified |
The 300-second window includes bursts, so these numbers describe saturation under that load pattern. They are not a baseline and not a production benchmark.
Why the division misses
The author compared the predicted throughput for each iSCSI session count against the measured sequential throughput. The results are below.
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| Counted iSCSI TCP connections | Measured (MB/s) | Predicted by the division (MB/s) | Measured ÷ predicted |
|---|---|---|---|
| 1 | 1,135.18 | 625 | 1.82x (prediction too low) |
| 2 | 1,135.18 | 1,250 | 0.91x (prediction slightly high) |
| 16 | 1,970.43 | 10,000 | 0.20x (prediction far too high) |
Two things stand out. First, going from one to two connections did not change the reported sequential throughput, so the second session added nothing in this test. Second, sixteen connections reached 1,970.43 MB/s, about a fifth of the 10,000 MB/s the linear model implied. A linear rule cannot capture both of those results, which is why it fails in both directions.
The author also notes that the same configuration measured differently in another environment, swinging by 2.64x. That is a reason to treat any single set of numbers, including these, as specific to its conditions.
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NVMe/TCP queues are not a simple count either
For NVMe/TCP, the tunable that matters is queue count rather than an iSCSI session setting. In the queue test, requesting 36 queues produced four effective submission/completion queues. The author explains that the effective number depends on what the host and transport negotiate, so the requested value is not what you should expect to get. Four is the observed outcome in that deployment, not a general ceiling for NVMe/TCP on FSx for ONTAP.
What the evidence does and does not establish
- The 5 Gbps (about 625 MBps) figure is an EC2 single-client reference point taken from AWS’s procedures. It is not a documented session-count formula.
- In one defined single-AZ test, the linear division was wrong for one, two, and sixteen iSCSI connections by different margins.
- The measurements do not establish a general replacement formula, a general FSx throughput ceiling, or a ranking of iSCSI over NFS or SMB. The author did not test protocol comparisons.
- Region, ONTAP version, client type, test duration, and the single-AZ topology all shape the outcome. Multi-AZ behavior was not verified.
- Compression state and inline storage-efficiency state were not recorded, so they cannot be ruled out as contributing factors.
- The author mentions placement-group status and network flow treatment as possible explanations for protocol behavior. Neither was tested, so treat them as open questions.
How to size a migration instead
- Define the workload in measurable terms. Record sustained throughput, peak throughput and how long peaks last, IOPS, I/O size, read/write mix, latency targets, and concurrency.
- Check the client path separately. Confirm the EC2 instance’s network bandwidth and whether the instance is limited to a single-client figure. The 5 Gbps reference applies to the client; it does not describe the file system.
- Set the target configuration. Pick the Region, AZ topology, throughput capacity, provisioned IOPS, and volume layout you plan to run in production.
- Configure protocol sessions or queues according to AWS’s procedures, then record what the host actually negotiates. For NVMe/TCP, check the effective queue count, not only the requested one.
- Measure at your I/O profile over a window longer than a burst. Use a load tool and parameters that match your workload, and report sustained numbers alongside peaks.
- Change one variable at a time. If results fall short, separate the client, network, file system, and volume limits before adding sessions or queues.
EBS planning is a separate constraint problem
If your target is Amazon EBS rather than FSx, the same kind of linear shortcut also fails. AWS’s EBS-optimized instance documentation says: “An instance’s EBS performance is bounded by the instance type’s performance limits, or the aggregated performance of its attached volumes, whichever is smaller.” The documentation also states that maximum IOPS and throughput limits depend on each other and on I/O size.
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In practice, check two numbers: the instance’s EBS capability and the combined performance of the volumes attached to it. Do not assume a volume’s advertised maximum is available through every instance. The EBS instance reference lists baseline and maximum throughput in MB/s at 128 KiB I/O and IOPS at 16 KiB I/O. Some instances can sustain maximum performance only for 30 minutes at least once in 24 hours, while others sustain it indefinitely. Check the footnotes for the instance family you choose, because these limits change over time.
When striping makes sense
Striping across several EBS volumes is an option when a single volume cannot meet your throughput, IOPS, size, or operational requirements. It is not an automatic answer to FSx session sizing. AWS’s SAP HANA guidance lists single-volume maximums of 1,000 MB/s for gp3 and 4,000 MB/s for io2 Block Express, and says most HANA deployments do not need striping on io2 Block Express. The guidance also recommends considering higher-performance volume types or settings before striping. Its example for a 4 TiB-memory system targets 1,250 MB/s on gp3 and suggests splitting across two 2,450 GiB volumes at 625 MB/s each. That example is specific to SAP HANA.
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Don’t confuse 625 MiB/s with 625 MBps
A similar number appears in AWS RDS documentation. For RDS for MySQL gp3 storage, setting IOPS to 40,000 requires storage throughput of at least 625 MiB/s. That is a storage performance rule for RDS, unrelated to the FSx client figure. The units also differ: MiB/s and MBps are not the same, so keep them distinct when you compare sources.
Bottom line for sizing
Do not use 625 MBps as a divisor for iSCSI sessions or NVMe/TCP queues. Measure the target configuration with a workload that matches yours, read the negotiated session and queue counts, and identify which limit (client, network, file system, or volume) is binding before changing anything.
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