Как гентушники делают билдсерверы?

BINPKG_FORMAT
    This variable sets the format used for tar binary packages. Possible values are xpak or gpkg.
    If BINPKG_FORMAT is set to xpak, packages will have the file extension .tbz2 for any compression type for historical reasons.
    If BINPKG_FORMAT is set to xpak, and FEATURES="binpkg-multi-instance" is set, packages will have the file extension .xpak.
    If BINPKG_FORMAT is set to gpkg, packages will have the file extension .gpkg.tar.

binpkg-multi-instance
    Enable support for multiple binary package instances per ebuild. Having multiple instances is useful for a number of purposes,
    such as retaining builds that were built with different USE flags or linked against different versions of libraries.
    The location of any particular package within PKGDIR can be expressed as follows:
            ${PKGDIR}/${CATEGORY}/${PN}/${PF}-${BUILD_ID}.xpak
    The build-id starts at 1 for the first build of a particular ebuild, and is incremented by 1 for each new build.
    It is possible to share a writable PKGDIR over NFS, and locking ensures that each package added to
    PKGDIR will have a unique build-id. It is not necessary to migrate an existing PKGDIR to the new layout, since portage is capable
    of working with a mixed PKGDIR layout, where packages using the old layout are allowed to remain in place.
    The new PKGDIR layout is backward-compatible with binhost clients running older portage, since the file format is identical,
    the per-package PATH attribute in the 'Packages' index directs them to download the file from the correct URI, and they automatically use BUILD_TIME metadata
    to select the latest builds.
    The eclean-pkg command can be used to prune old builds from PKGDIR. It is also possible to remove packages manually, and then run aqemaint --fix binhost'
    to update the ${PKGDIR}/Packages index. This feature is enabled by default.