OSTree Static Deltas

OSTree is implemented as a Content Addressable Storage system inspired by Git. Both systems organize their objects in a tree-based hierarchy. The client’s job is to work through these trees of objects and apply the correct changes. While Git has smart protocol, OSTree does not. This can lead to certain types of OTAs being really inefficient — the client will be requesting a large number of files via HTTP requests.

OSTree has a solution for this problem called static deltas. OSTree can produce static deltas that are good balance between number of files to download and the size of each file. For instance, a 1.5G OTA might be split up into about 38 files that are each about 30Mb.

Generating Static Deltas

Fioctl includes a command to help generate static deltas. Since these deltas can sometimes require a bit of processing power and network bandwidth, the actual work is performed as CI Job in the Factory.

Understanding Why Static Deltas are Needed

An operator will be planning an update to a new Target. This Target will have been produced by CI and already have a tag that was used by CI devices. For example, the targets.json might include:

"raspberrypi4-64-lmp-9" : {
  "custom" : {
    "version": "9",
    "tags" : ["main"],
 ...

In this example, Target #9 has passed CI and needs to be deployed to devices following the promoted tag. The operator can determine what static deltas are needed by running:

fioctl targets static-deltas --dryrun --by-tag promoted 9
Dry run: Would generated static deltas for target versions:
7 -> 9
5 -> 9

In this case, Fioctl has looked at all the devices configured to the promoted tag, and found the Target versions #5 and #7. To produce the most efficient OTAs, two static deltas need to be created.

Creating Static Deltas

Running the same command without --dryrun will produce the static deltas via a CI Job:

fioctl targets static-deltas --by-tag promoted 9

Once the static deltas are in place, Target #9 can be re-tagged so that “promoted” devices will apply the update:

fioctl targets tag --tags main,promoted --by-version 9

When the CI job completes, devices on the promoted tag will start performing OTA updates that will use the static deltas.