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Garbage collection

Static vs dynamic indexing

Static indexing is used for base packages. At pkgfs startup, base packages are pre-populated in /pkgfs/packages based on the static_packages index located in /system/data/static_packages. These static packages are then used to bootstrap the system, so that core components like pkg_resolver, pkg_cache, netstack, sysmgr can be started.

pkgfs doesn't maintain state across reboots but the base package set is guaranteed to always be present. Base packages cannot be deleted.

The dynamic index stores a mapping of all ephemerally fetched packages. pkgfs will pre-populate the dynamic index with any present packages (i.e. meta.far and all BLOBs resolved) listed in /system/data/cache_packages. In memory, the dynamic index has the most recently resolved version of a package with the same name by keying on the $name/$variant of the package. pkgfs then "forgets" about the old version of the package. The old version of the package is still present in the system but no longer referenced. The dynamic index is then used to implement garbage collection.

How to garbage collect

There is no notion of installing a package in fuchsia and likewise no notion of deleting a package. Rather, garbage collection can be thought of as a means to reclaim space. Garbage collection can be triggered manually by running pkgctl gc or it can be triggered by the system-updater. The implementation of garbage collection uses the fuchsia.space/Manager protocol. The system-updater trigger happens twice; once before a system update and once after fetching the update package.

The pkgfs garbage collector currently uses set differences to determine which packages are live packages. A package is considered live if any of the following is true:

  • A package is a base package in the static index.
  • A package is in the process of being updated (by tracking the meta.far merkle root and any missing BLOBs until they’ve been fully resolved).
  • A package is the most recently resolved version of an ephemeral package according to its meta or package in the dynamic index.

When garbage collection runs, it deletes every BLOB in blobfs that is not referenced by a live package.

Known issues

Existing garbage collection implementation is suboptimal.

  • An old version of an ephemeral package that is open can be garbage collected. This may lead the garbage collector to erase a package out from under a component.

  • If system-updater fails to download a new package, the garbage collector protects both the base package and the most recent package version, which leads to duplicate copies of every package. If this happens, you should reboot the Fuchsia device to clear the list of activated packages.