Azure SQL Database sells two licensing and capacity models that look interchangeable on the price calculator and are not. The DTU model bundles compute, memory, and IO into a single capacity unit — simple to size, impossible to optimise, no Azure Hybrid Benefit, no reservations on most tiers. The vCore model exposes compute and storage independently, supports Azure Hybrid Benefit on Windows-licensed SQL Server SA, supports vCore reservations for 33–55% commitment discounts, and supports Hyperscale and Business Critical service tiers that DTU does not. The azure sql licensing answer for any workload above ~$2K/month is vCore. DTU survives only as the path of least resistance for development databases and small departmental apps.
The DTU model and what it actually buys you
The DTU (Database Transaction Unit) model is Azure SQL Database's original licensing posture, dating to the service's launch. A DTU is a blended unit: 1 DTU buys you a fixed slice of CPU, memory, log write throughput, and read IOPS. You pick a service tier (Basic, Standard, Premium) and a DTU count; the price is all-inclusive. There is no reservation, no Hybrid Benefit, no separation of compute from storage. The model is the easiest to provision and the hardest to optimise.
For workloads under approximately $1,500 per month per database, DTU is functionally fine. For anything larger, DTU costs 25–55% more than the equivalent vCore configuration once you apply the commitment and AHB levers that DTU does not support. Microsoft maintains DTU primarily for transition compatibility and small-customer simplicity; the strategic direction is vCore.
The vCore model and why it wins above the threshold
The vCore model separates compute from storage and exposes both as standard Azure resources. You pick a hardware generation (Standard-series, Memory-Optimised, Compute-Optimised), a vCore count (compute power), a memory ratio, and a storage tier separately. The model unlocks four economic levers DTU cannot reach:
- Azure Hybrid Benefit on SQL Server. If you hold Windows-licensed SQL Server Enterprise or Standard with active Software Assurance, you can apply AHB to your vCore SQL Database, removing the SQL licence component from the hourly price. Savings: 30–55% on the vCore line item.
- Reserved capacity. vCore SQL Database supports one- and three-year capacity reservations, providing 33–55% off pay-as-you-go for committed vCore counts.
- Service tier choice. vCore exposes Business Critical (always-on, premium IO) and Hyperscale (separated compute/storage, near-instant scale, >100TB databases) service tiers that DTU does not offer.
- Independent compute/storage scaling. You can scale compute up and down independently of storage, including pausing the Serverless tier overnight on dev databases.
| Lever | DTU model | vCore model |
|---|---|---|
| Azure Hybrid Benefit | Not supported | Supported (saves 30–55%) |
| Reservations | Not supported (most tiers) | Supported (33–55% off) |
| Hyperscale tier | No | Yes |
| Business Critical tier | No | Yes |
| Serverless / pause | No | Yes (General Purpose only) |
| Compute/storage separation | No | Yes |
| Transparent pricing | Bundled | Itemised |
Microsoft Cost Management does not flag DTU-to-vCore migration opportunities. Microsoft's account team will not flag them either — recommending vCore plus AHB plus reservations cuts the SQL line item by 40–55% and reduces the buyer's MACC burn rate. The opportunity is structural, lasting, and the buyer has to find it themselves.
DTU-to-vCore migration mechanics
The migration is administratively low-risk and reversible. You change the service tier on the existing database from a DTU SKU (e.g., S6, P4) to the equivalent vCore SKU (e.g., GP_Gen5_8, BC_Gen5_8). Azure handles the SKU change online with no data movement; the database stays up. There is no rebuild, no export/import, no application change. The change takes 10–60 minutes depending on database size and tier.
The right sizing translation is approximate but well-mapped: 100 DTUs ≈ 1 vCore in Standard tier; 125 DTUs ≈ 1 vCore in Premium tier. The Azure portal does the conversion automatically. After the migration, apply AHB on the new vCore database (single toggle in the portal) and purchase a vCore reservation for the steady-state capacity.
Service tier decisions inside vCore
vCore offers three service tiers, each suited to a different workload profile:
- General Purpose. Default tier. Compute and storage co-located, remote durable storage, suitable for 80% of OLTP workloads. Supports Serverless (auto-pause on idle).
- Business Critical. Premium IO, always-on local SSD, built-in read replica, suitable for latency-sensitive transactional workloads. 2–2.5x the General Purpose cost; pays back for workloads where IO latency drives application performance.
- Hyperscale. Decoupled compute/storage, near-instant scale, supports databases >100TB, fast point-in-time restore. Suitable for very large databases and bursty analytical workloads. Compute scaling is independent of storage capacity.
The tier selection is workload-dependent, not commercial. Wrong tier costs more, but the more common error is staying in DTU because the migration to vCore feels like a bigger change than it actually is.
vCore reservation strategy
vCore reservations commit you to a specific vCore count, hardware generation, and service tier for one or three years. Discount: 33–55% off pay-as-you-go on the vCore component. Reservations apply automatically to matching usage — there is no per-database configuration to attach.
The reservation strategy mirrors compute: cover your steady-state baseline with three-year reservations, cover the predictable variable with shorter or smaller reservations, take pay-as-you-go on the volatile top. For Hyperscale, only the compute component is reservable; storage is always pay-as-you-go.
Anonymised case study: $980K SaaS migration
A SaaS client running a 47-database Azure SQL Database tenant on DTU Premium tier was paying $1.78M annually. We re-modelled the workload as vCore General Purpose and Business Critical (per workload class), applied AHB against their existing SQL Server Enterprise SA inventory, and layered three-year vCore reservations on the steady-state baseline. Net annual SQL spend reduced to $800K — $980K saved against the prior DTU posture. The migration completed over two weekends with no application change and no downtime longer than the per-database 25-minute SKU switch. The saving was structural — it recurs every year.
Where to take this from here
If you run Azure SQL Database at meaningful scale on DTU pricing, the migration to vCore is the single highest-ROI Azure database action available to you. The migration is low-risk, reversible, and unlocks AHB and reservations — both of which are unavailable to DTU. For the broader Azure cost picture, the complete Azure cost optimisation guide places SQL alongside compute, storage, and networking decisions. For commitment strategy across the full stack, the Savings Plans vs RIs guide covers the compute-side equivalents. For Microsoft renewal context, the EA tier collapse playbook shows how SQL Server SA inventory becomes a commercial lever in EA renewals.