Ebuild repository

An overlay is an additional repository that Portage takes into account when dealing with software.

Within Gentoo Linux, users already have one "main" package repository, called the Portage tree. This main repository contains all the software packages (called ebuilds) maintained by Gentoo developers. But users can add additional repositories to the tree that are "layed over" the main tree - hence the name, overlays.

Since package repositories are nothing more (or less) than a set of files (ebuilds, metadata files, ChangeLog entries ...) these repositories can be pulled in from public repositories (git, cvs, svn ...) or downloaded as tarballs and extracted manually on the system. It is however well advised to use managed repositories by trusted third parties as any installed overlay will cause Portage to look into those files as well when deciding which software to install.

Treatment of overlays
Portage uses the  variable to link to the installed overlays on the system. This variable uses a space-delimited list of paths on the system where Portage can find the roots of the additional repositories.

Manually setting overlay locations
When you want to create an overlay yourself, create a location (say ) in which you will put the packages you want Portage to look into as well. Then add  to your /etc/make.conf file:

Using crossdev
crossdev will automatically place the ebuilds/categories it generates into the first overlay found in PORTDIR_OVERLAY. If you want to prevent crossdev from disturbing layman overlays or your host's personal overlay, you should create an overlay (normally created at ) specifically for crossdev's use.

Using layman
To make management of multiple overlays simple, a tool called layman is developed. This tool knows about popular user- and developer managed overlays and is able to install & synchronize them as well as add them to the  location.

Overlay priorities
Each overlay has its unique priority. This makes sure that in the case of a specific version being found in several overlays, the resolution is unambiguous. Ebuilds from overlays with higher priorities take precedence over ebuilds from overlays with lower priorities.

The list of overlays with their priorities can be obtained through the output of the following command

Unless the  variable has been modified as described below, the default gentoo portage tree will have a priority of -1000. That means that all other overlays take precedence. That is the default behavior, because overlays are designed to "lie over/on top" of the portage tree.

Setting overlay priorities
The overlay priority is calculated from the order of overlay entries in the  variable. Portage "walks" through the variable from left to right and increments the priority on the way. The leftmost entry starts with a priority of 1, the next entry has a priority of 2 and so on.

Unless the  contains the portage tree entry, the portage tree will always be assigned a priority of -1000. This can be easily changed by putting  in the   variable:

In the example above the user overlay will be assigned a priority of 1 and the portage tree will be assigned a priority of 2.

Cache generation
When large overlays are installed, portage may take a long time to perform operations like dependency resolution. This is because overlays do not usually contain a metadata cache.

You may generate a local metadata cache by running emerge --regen after syncing your overlays.

eix-sync
eix-sync can run emerge --regen after syncing your overlays and portage tree.

eix-update
eix-update can utilize the metadata cache generated by emerge --regen for a speedup and better accuracy. To enable this, set the OVERLAY_CACHE_METHOD to "assign" in /etc/eixrc.