Build Caches

Some sites may encourage users to set up their own test environments before carrying out central installations, or some users may prefer to set up these environments on their own motivation. To reduce the load of recompiling otherwise identical package specs in different installations, installed packages can be put into build cache tarballs, pushed to your Spack mirror and then downloaded and installed by others.

Whenever a mirror provides prebuilt packages, Spack will take these packages into account during concretization and installation, making spack install significantly faster.


We use the terms “build cache” and “mirror” often interchangeably. Mirrors are used during installation both for sources and prebuilt packages. Build caches refer to mirrors that provide prebuilt packages.

Creating a build cache

Build caches are created via:

$ spack buildcache push <path/url/mirror name> <spec>

This command takes the locally installed spec and its dependencies, and creates tarballs of their install prefixes. It also generates metadata files, signed with GPG. These tarballs and metadata files are then pushed to the provided binary cache, which can be a local directory or a remote URL.

Here is an example where a build cache is created in a local directory named “spack-cache”, to which we push the “ninja” spec:

$ spack buildcache push ./spack-cache ninja
==> Pushing binary packages to file:///home/spackuser/spack/spack-cache/build_cache

Note that ninja must be installed locally for this to work.

Once you have a build cache, you can add it as a mirror, discussed next.

Finding or installing build cache files

To find build caches or install build caches, a Spack mirror must be configured with:

$ spack mirror add <name> <url or path>

Both web URLs and local paths on the filesystem can be specified. In the previous example, you might add the directory “spack-cache” and call it mymirror:

$ spack mirror add mymirror ./spack-cache

You can see that the mirror is added with spack mirror list as follows:

$ spack mirror list
mymirror           file:///home/spackuser/spack/spack-cache

At this point, you’ve create a buildcache, but spack hasn’t indexed it, so if you run spack buildcache list you won’t see any results. You need to index this new build cache as follows:

$ spack buildcache update-index ./spack-cache

Now you can use list:

$  spack buildcache list
==> 1 cached build.
-- linux-ubuntu20.04-skylake / gcc@9.3.0 ------------------------

With mymirror configured and an index available, Spack will automatically use it during concretization and installation. That means that you can expect spack install ninja to fetch prebuilt packages from the mirror. Let’s verify by re-installing ninja:

$ spack uninstall ninja
$ spack install ninja
==> Installing ninja-1.11.1-yxferyhmrjkosgta5ei6b4lqf6bxbscz
==> Fetching file:///home/spackuser/spack/spack-cache/build_cache/linux-ubuntu20.04-skylake-gcc-9.3.0-ninja-1.10.2-yxferyhmrjkosgta5ei6b4lqf6bxbscz.spec.json.sig
gpg: Signature made Do 12 Jan 2023 16:01:04 CET
gpg:                using RSA key 61B82B2B2350E171BD17A1744E3A689061D57BF6
gpg: Good signature from "example (GPG created for Spack) <>" [ultimate]
==> Fetching file:///home/spackuser/spack/spack-cache/build_cache/linux-ubuntu20.04-skylake/gcc-9.3.0/ninja-1.10.2/linux-ubuntu20.04-skylake-gcc-9.3.0-ninja-1.10.2-yxferyhmrjkosgta5ei6b4lqf6bxbscz.spack
==> Extracting ninja-1.10.2-yxferyhmrjkosgta5ei6b4lqf6bxbscz from binary cache
==> ninja: Successfully installed ninja-1.11.1-yxferyhmrjkosgta5ei6b4lqf6bxbscz
Search: 0.00s.  Fetch: 0.17s.  Install: 0.12s.  Total: 0.29s
[+] /home/harmen/spack/opt/spack/linux-ubuntu20.04-skylake/gcc-9.3.0/ninja-1.11.1-yxferyhmrjkosgta5ei6b4lqf6bxbscz

It worked! You’ve just completed a full example of creating a build cache with a spec of interest, adding it as a mirror, updating its index, listing the contents, and finally, installing from it.

By default Spack falls back to building from sources when the mirror is not available or when the package is simply not already available. To force Spack to only install prebuilt packages, you can use

$ spack install --use-buildcache only <package>

For example, to combine all of the commands above to add the E4S build cache and then install from it exclusively, you would do:

$ spack mirror add E4S
$ spack buildcache keys --install --trust
$ spack install --use-buildcache only <package>

We use --install and --trust to say that we are installing keys to our keyring, and trusting all downloaded keys.


Initial build and later installation do not necessarily happen at the same location. Spack provides a relocation capability and corrects for RPATHs and non-relocatable scripts. However, many packages compile paths into binary artifacts directly. In such cases, the build instructions of this package would need to be adjusted for better re-locatability.

spack buildcache

spack buildcache push

Create tarball of installed Spack package and all dependencies. Tarballs are checksummed and signed if gpg2 is available. Places them in a directory build_cache that can be copied to a mirror. Commands like spack buildcache install will search Spack mirrors for build_cache to get the list of build caches.




list of partial specs or hashes with a leading / to match from installed packages and used for creating build caches

-d <path>

directory in which build_cache directory is created, defaults to .


overwrite .spack file in build_cache directory if it exists

-k <key>

the key to sign package with. In the case where multiple keys exist, the package will be unsigned unless -k is used.


make paths in binaries relative before creating tarball


answer yes to all create unsigned build_cache questions

spack buildcache list

Retrieves all specs for build caches available on a Spack mirror.




list of partial package specs to be matched against specs downloaded for build caches

E.g. spack buildcache list gcc with print only commands to install gcc package(s)

spack buildcache install

Retrieves all specs for build caches available on a Spack mirror and installs build caches with specs matching the specs input.




list of partial package specs or hashes with a leading / to be installed from build caches


remove install directory if it exists before unpacking tarball


answer yes to all to don’t verify package with gpg questions

spack buildcache keys

List public keys available on Spack mirror.




trust the keys downloaded with prompt for each


answer yes to all trust all keys downloaded