Firmware deltas

How NervesHub delivers bandwidth-efficient partial firmware updates: what a delta is, how it's generated, and what devices need to use them.

Written By Josh Kalderimis

Last updated About 2 months ago

A firmware delta is a partial update containing only the difference between two firmware versions, instead of a full image. Devices download far less data, which matters a lot on cellular and other constrained or metered networks.

Why deltas

A full .fw image can be tens of megabytes. If a release only changes part of the system, most of that download is bytes the device already has. A delta ships just the changed blocks, so a small change becomes a small download. Nothing about the safety guarantees changes: the resulting firmware is still signed and verified the same way.

How they're generated

When a device is due to update from a known source firmware to a target firmware, NervesHub can generate a delta between the two using fwup and xdelta3:

  • A delta is tied to a specific source → target pair.

  • Generation happens on the server. Each delta has a status while it's produced: processing, then completed, failed, or timed_out.

  • NervesHub tracks each delta's size against the source and target so you can see the savings.

If a delta can't be produced for a given pair, NervesHub falls back to sending the full firmware, so updates still succeed.

What's required

Deltas need cooperation from a few places:

  • The firmware must be delta-updatable. This is a property of how the firmware is built; NervesHub records whether a given firmware supports deltas.

  • The device's tooling must support it. Applying a delta needs a compatible fwup on the device. NervesHub tracks the tool version a delta requires.

  • The deployment group must allow deltas. Deployment groups have a delta updatable setting (on by default). See Deployment groups & releases.

When all three line up, matching devices receive a delta; otherwise they receive the full image.

Trade-offs

  • Big win: much smaller downloads, especially for incremental releases over expensive networks.

  • Cost: deltas are generated per source/target pair and take server-side work to produce. The first device updating from a new source may wait for generation; later devices on the same pair reuse the completed delta.

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