RFC-0144: Size Check Tool

RFC-0144: Size Check Tool
  • Developer

Proposes a rewrite of size_checker.go to create a more cohesive in-tree and out-of-tree developer experience.

Gerrit change
Date submitted (year-month-day)2021-11-22
Date reviewed (year-month-day)2021-12-10


This document proposes the development of a new size check tool that will be added to the SDK, which allows developers and product owners to verify that their packages and products do not exceed specified size budgets. While the implementation details are intentionally excluded, because out-of-tree Assembly is continually shifting the foundation, design requirements and goals are mentioned.


During software development, it is helpful to know whether your software fits within the size constraints of your target. Fuchsia has a size check tool that analyzes the space consumed by different groups of software, and enforces a budget for each group. This tool is tightly bound to the implementation details of fuchsia.git, therefore it cannot be used without a fuchsia.git checkout. As a result, consumers of Fuchsia, such as Chromium, have implemented their own ad-hoc method for checking the size of their software. By rewriting the size check tool as a ffx plugin and adding it to the Fuchsia SDK, the method of enforcing size budgets on Fuchsia software will be unified, making it easier for future developers to get started with Fuchsia.


Who has a stake in whether this RFC is accepted.


  • Hunter Freyer (hjfreyer@google.com)


  • Saman Sami (samans@google.com) - Functionality
  • Anthony Fadrianto (atyfto@google.com) - Integration with Infrastructure
  • Sébastien Marchand (sebmarchand@google.com) - Integration with Infrastructure
  • Rohan Pavone (rohpavone@google.com) - Integration with Chromium
  • Amit Uttamchandani (amituttam@google.com) - Integration into ffx


  • Aaron Wood (aaronwood@google.com) - Assembly Integration


This project was socialized as an internal document and discussed in meetings with the stakeholders ahead of time. The requirements were already gathered before this RFC was drafted.


This RFC assumes an understanding of the following topics.

Two flavors of size checking

Size checking can be separated into two categories:

  1. Images do not exceed their target partition size when flashing.
  2. Blobs fit within the FVM budget during updates.

Flashing: A developer can flash new images onto a Fuchsia target. When an image is flashed, the contents of the partition is completely replaced by the bytes of the new image. To ensure a successful flash, the new images must not be larger than the size of the partitions.

Updating: A Fuchsia target can take a update Over-The-Air. To ensure a successful update, the total space consumed by blobs must not exceed the budget set for the FVM. Often times, space in the FVM is reserved for an UpdatePackage and additional blobs, which means that the tool cannot simply fill the FVM then check that the final image fits on the partition.

Current tools

Fuchsia supplies two tools that assist in size checking:

  • size_checker.go
  • blobfs-compression


Link to source

This tool can be used in-tree (inside fuchsia.git) to ensure blobs fit within their budget. size_checker.go does this by determining the compressed and aligned size of a directory of blobs in the Fuchsia repository. The budgets can be defined per-product, but are currently not used for any products in fuchsia.git. Because it specifically checks directories in fuchsia.git, the tool is incapable of being used without a fuchsia.git checkout.

First, the Fuchsia build produces a blobs.json file that lists the compressed and aligned size of every blob in blobfs, and a blob.manifest file that lists the merkle and source path of those blobs.

Next, size_checker.go is run and performs the following:

  1. Reads blobs.json to collect the compressed size of every blob: compressed_size.
  2. Reads blob.manifest to collect all packages.
  3. Calculates the number of times a blob is included in a package: share_count.
  4. Calculates the shared size of each blob: shared_size = compressed_size / share_count. Because multiple budgets could include the same blob, we use the shared_size of a blob to distribute the size consumption evenly across the budgets.
  5. Constructs an N-ary sum tree representing the space consumed by every directory in the build output by adding each blob's shared_size to the nodes in the blob's path.
  6. Iterates over each budget, and asserts that the size budgeted for a directory is larger than the space consumed.
  7. Iterates over each non-blobfs package in the budgets, uses blobfs-compression to calculate the sum of the blob sizes, and asserts that it falls within the budget.
  8. Prints the results.

Sum Tree


Link to source

This tool can estimate the compressed and aligned size of a set of blobs, but is not 100% accurate, and may occasionally over-estimate the size of a blob. The tool will never under-estimate the size of a blob.

blobfs-compression is delivered to clients via the Fuchsia SDK, and is currently the only method of estimating the size of code built for Fuchsia that exists outside of fuchsia.git. Chromium is a client of this tool.

Problems with current tools

  • In-Tree and Out-Of-Tree methods for checking package sizes are different tools.
  • Neither tool can directly check the size of images to ensure flashing succeeds.
  • size_checker.go
    • Is not documented.
    • Depends on the implementation details of the Fuchsia build, which is not a stable contract.
    • Does not work out-of-tree.
  • blobfs-compression
    • Is not guaranteed to return the accurate compressed size of a blob.
    • Operates at a blob-level, which is not as useful as operating at a package-level. Developers add software to a product on a per-package basis.
    • Does not have a standard output format, therefore it is not easily scriptable, and automated usage is fragile.


  1. Provide a tool or tools in the SDK that work both In-Tree and Out-Of-Tree.
  2. The tool(s) support all desired existing use cases of the current tools.
  3. The tool(s) can perform two types of size checks:
    • (a) Images
    • (b) Blobs in a set of packages
  4. In case (2), blob deduplication across the set of all packages in all sets will be performed.
  5. The tool(s) will return a failure when a user-specified budget is exceeded.
  6. The tool(s) work together nicely with the other SDK tools, such as the Image Assembler (RFC-0072).
  7. The tool(s) can be easily used in scripts, with parsable output, and produces the output used by the Gerrit Size Plugin.
  8. The tool(s) usage and architecture will be documented in fuchsia.dev.


Cohesive Developer Experience

A new ffx plugin will be written in Rust that performs the two types of size checks. Incorporating it into ffx is preferable, as per the CLI Rubric, to encourage a shared workflow, and increase the discoverability. Also, preference will be given to expanding existing file formats, rather than inventing new ones.


This tool will primarily be used in build systems, therefore the output should be parsable. The output format will likely use json5 as the output format, but as the Assembly project progresses, alternate formats may be considered.

Image Sizes

The image size checker should be run at the same time as the construction of the flash manifest, where the assembled images are mapped to partitions. Currently, flash manifest creation is done in the Fuchsia build system, so the ffx plugin will be invoked via a GN action. In the future, a SDK tool may be provided for constructing the flash manifest, and as a ffx plugin, our size checker tool should work seamlessly in that scenario.

Measuring the size of an image is a simple as opening the file and reading the length in the metadata. This works for any images that are not compressed or sparse. An example of where this method does not work is sparse FVMs, and because there currently exists no library for calculating the expanded size of a sparse FVM, the tool will ignore size checks for sparse FVMs.

Package Sizes

Blob and package size checks should be closely integrated with Assembly, because it is during Assembly that engineers group packages into sets and combine these sets to specify products.

The size check tool uses blobs.json files that list the compressed and aligned sizes of each blob. The developer may provide multiple blobs.json files as an input to the tool. If a package's blob is not found in any provided blobs.json files, then the size check tool will generate a single blobfs image for all the missing blobs, using the blobfs tool, which generates an additional blobs.json. The size check tool will read the generated blobs.json to get the sizes of the missing blobs.

Similar to the existing size_checker.go tool, the new size check tool will calculate the shared size of each blob by dividing the size by the number of packages that use the blob. This shared size is what is used during budgeting.


The implementation and integration of this tool will be completed in several stages.

  1. Declare the budgets for the package sets. See the explanation below.
  2. Write a ffx plugin that ensures the package sets fall within their budget
  3. Ensure that the output of the ffx plugin matches that of the previous tool as part of the build system. If the outputs are different, then the build should fail. See the explanation below.
  4. Declare the budgets for the images
  5. Add functionality to ensure the image sizes fall within their budgets
  6. Disable size_checker.go and make the new ffx plugin load bearing
  7. Delete the size_checker.go code.

Budget file generation

In order to avoid the toil of maintaining two identical sets of budgets for a given product, the build system will be updated to read the existing size_limits.json and assembly product config and produce the new budgets file. The product config will be parsed to collect every package manifest, and those manifest will be categoried according to their directory budgets set in size_limits.json.

A potential budget file format is:

        name: "name-of-package-set",
        packages: [
        budget_bytes: 12000,

Once the new size check tool is load-bearing for the build, the generated budgets file will be checked into the repository and the generated code will be deleted.

Output comparison

During the transition to the new size checker, the build system will assert that the output of the new tool matches that of the new tool. A script will be written that parses the outputs of both tools, and ensures that the budgets and calculated consumption for each package group (or directory) is equivalent. In order to correlate the two output formats, the script will assume that the names identifying the sets of packages are identical. Because both the old tool and the new tool are using the same underlying mechanism for calculating the size of a blob (using blobfs), the calculated consumption will also be identical.


The size check tool is intended for use in a build system, therefore is has a higher tolerance for performance instability than, for example, a network stack. On the other hand, lowering build times is important for developer productivity. Additionally, this tool is on the critical path for a build, because it must wait for all the packages to be build, and the images to be assembled before it can start running, therefore it is not parallelizable.

The current size_checker.go tool takes approximately 1-2 seconds to run, and our new tool should aim to be close to the same duration.


See the Design section.

Backwards Compatibility

This design does not need to be backwards compatible.

Security considerations

This design has no security implications.

Privacy considerations

This design has no privacy implications.


In the Fuchsia build, the new size check tool will be ran alongside size_checker.go, and the output will be compared to ensure that they have the same budgets and calculated space consumption.

Unit tests will also be written as features are added to the tool.


Documentation will be added to fuchsia.dev.

Drawbacks, alternatives, and unknowns

There exist a few unfavorable alternatives.

Do nothing: This would require clients to create their own ad-hoc method for completing size checks, which does not scale well as our client base increases.

Refactor existing tools: size_checker.go could be refactored to be supported out-of-tree. According to the CLI Rubric, the preference is that public tools are added to ffx to encourage a shared workflow and better discovery. Also, the changes required to refactor the current code are so drastic that it is actually less work to rewrite the tool entirely.

Use blobfs-compression: In order to calculate the compressed and aligned size of each blob, the blobfs-compression tool could be used instead of generating a new blobfs image. This alternative has the major drawback, that the blobfs-compression tool is not guaranteed to have an accurate compression or alignment. To be specific the blobfs-compression tool assumes non-compact merkle trees, which often causes it to result in a larger blob size than reality. Additionally, the blobfs-compression tool can only measure one blob at a time, and is therefore more cumbersome to use, and likely slower than running the blobfs tool once. Generating a blobfs image is more inline with the current size_checker.go and more accurate.

Prior art and references

See the Background section.