SSD Class-range

Audience: Sales, Procurement, Production.

Purpose: Standardise on SSD capacities range-class

Problem: Determine whether 250 GB vs 256 GB (and likewise 500 vs 512, 1000 vs 1024) are materially different for our business use, and define a clear procurement rule that balances flexibility with SOW compliance. 

Capacity range-classes 

Treat the following as equivalent for quoting and procurement, unless a contract requires an exact label:

·        ≈ 256 GB class: 250 GB ↔ 256 GB (equivalent)

·        ≈ 512 GB class: 500 GB ↔ 512 GB (equivalent)

·        ≈ 1 TB class: 1000 GB ↔ 1024 GB (equivalent)

Note: 240/480/960 GB are a smaller class (≈6.7% less than 256/512/1024). Only use if the SOW/quote says “≥240/480/960 GB” or the customer approves.

Summary

·        250 GB and 256 GB are the same class. 256 GB gives ~2.4% more usable space (about 5.6 GiB).

·        Same idea for 500 vs 512 and 1000 vs 1024.

·        What matters more than the label: 

• NAND type (prefer TLC)

• Controller

• Endurance (TBW)

• Warranty

• Real‑world performance.

·        Customer line: “250 GB and 256 GB are the same class—256 GB just has about 2% more room. Performance and reliability depend on the model, not the number on the box.”

Why labels differ

1. Decimal vs binary math

o   Vendors print in decimal GB (1 GB = 1,000,000,000 bytes).

o   Windows shows GiB (1 GiB = 1,073,741,824 bytes).

2. Over‑provisioning (OP)

o   SSDs keep some space hidden for wear‑levelling and firmware. Different models reserve different amounts.

3. Raw NAND vs user capacity

o   Many “250 GB” and “256 GB” models may use the same raw NAND, but expose slightly different user capacity.

How to explain it simply

·        It’s just naming and rounding. 256/512/1024 line up with power-of-two steps; 250/500/1000 are round numbers.

·        Both use the same math under the hood. After formatting, the OS reports slightly less than the label either way, or the gap between 250↔256 (etc.) is only around 2–3%.

·        Real-world use: No noticeable speed or lifespan change just because of 250 vs 256 (or 500 vs 512, 1000 vs 1024). Pick based on brand, controller, NAND type (TLC vs QLC), warranty, and TBW, not the tiny label difference.

What matters more than the capacity differences

When selecting models, prioritise these:

1. NAND type: Prefer TLC for business use (predictable endurance/perf). Use QLC only when cost/usage fits (read‑heavy, low write).

2. Endurance (TBW): Set internal minimums (guide):

o   ~≥150 TBW for ≈256 GB, ≥300 TBW for ≈512 GB, ≥600 TBW for ≈1 TB.

o   Higher is better; check warranty alignment.

3. Controller + DRAM: Business models typically use proven controllers and include DRAM (better sustained perf & low write‑amp).

4. Warranty & support: Target 3–5 years with clear RMA terms.

5. Firmware maturity: Prefer models with stable firmware and enterprise/commercial track record.

Sales talking points (for reference)

·        Simple version:
“250 GB and 256 GB are the same class. 256 GB gives about 2% more usable space. The reliability and speed depend on the specific model, not this small size difference.”

·        If asked “where did the 6 GB go?”
“Storage brands use decimal units (GB), while Windows shows binary (GiB). The rest is reserved for healthy SSD operation (wear‑levelling and firmware).”

·        If customer needs exact numbers:
“On Windows, 250 GB shows ~232.8 GiB and 256 GB shows ~238.4 GiB.”

Procurement policy (capacity)

1. Allowable substitutions:

o   250 ↔ 256 GB (same class)

o   500 ↔ 512 GB (same class)

o   1000 ↔ 1024 GB (same class)

2. Do not auto‑substitute:

o   240 for 250/256, 480 for 500/512, 960 for 1000/1024 — needs customer agreement or SOW that says “≥240/480/960 GB.”

3. Quote phrasing:

o   Use “≈256 GB class (250–256 GB)”, “≈512 GB class (500–512 GB)”, “≈1 TB class (1000–1024 GB)”.

4. Spec checklist on PO:

o   Capacity class (as above)

o   Form factor & interface (e.g., M.2 2280 NVMe Gen3/Gen4; 2.5” SATA)

o   NAND: TLC (unless QLC explicitly approved)

o   Min TBW threshold (see guide)

o   DRAM‑based (preferred)

o   Warranty term (3–5 years)

o   Approved controller families (if applicable)

FAQ

 Q: Does 256 GB run faster than 250 GB?
A: Not because of the label. Speed depends on controller, NAND, DRAM cache, and firmware.

 Q: Why does Windows show less than the box?
A: Decimal vs binary units + reserved space for SSD health.

Q: Can we promise 256 GB exactly?

A: Yes, if required by contract—but 250 and 256 are our default equivalent class.