This recipe describes how to build a CentOS image using Singularity, with special emphasis for Ubuntu compatible host.
NOTE: this tutorial is intended for Singularity release 2.2, and reflects standards for that version.
In theory, an Ubuntu host can create/bootstrap a CentOS image by installing the
yum package, which is a front-end controller for RPM. In order for this to work on Ubuntu, a software called Berkeley DB must be identical in version to the version expected by
yum. Unfortunately, these two verisons tend to be different, and this situation poses a difficult challenge for Singularity to deal with. A perfectly working
centos.def file that can bootstrap a CentOS image from a RHEL-compatible host will not work when executed on Ubuntu, yielding the following error:
YumRepo Error: All mirror URLs are not using ftp, http[s] or file. Eg. Invalid release/ removing mirrorlist with no valid mirrors: /var/cache/yum/x86_64/$releasever/base/mirrorlist.txt Error: Cannot find a valid baseurl for repo: base ERROR: Aborting with RETVAL=255
The error above results during the bootstrap process, and happens because Ubuntu is trying to use its version of Berkeley DB to create the RPM database in the CentOS image. Because of a version conflict, subsequent use of
yum fails because it is unable parse the Berkeley DB.
This problem is not exclusive to Ubuntu. Other flavors of Linux likely have the same problem. In fact, building a CentOS image hosted by a newer CentOS host results in the same problem!
There are a number of solutions:
db*_loadthat match the Berkeley DB version for the version of CentOS being imaged, and add a conversion step during the Singularity bootstrap process.
- Perform a double bootstrap process: First build a base container containg CentOS (e.g. import from docker) and then 2) use this image to build the final desired CentOS image. You can run a container from within another container with Singularity as long as you are root when you do it.
- Go to a CentOS machine and create a basic singularity image, and copy this image to the Ubuntu machine. Since such an image already has working
yumcommands, and an RPM database with the correct version of Berkeley DB, a subsequent
singularity bootstrapon this image can successfully run
yumto update and add additional software to this image.
singularity import centos.img docker://centos:6to seed the CentOS image.
- Import the container from Singularity Hub, when this feature becomes available.
Create an image on CentOS (Option 3)
- Identify a CentOS machine with the same major version of CentOS you want to build. Don’t use a CentOS-7 machine to build a CentOS-6 machine, because it won’t work. (Building a CentOS-7 image on a CentOS-6 host works, but the RPM DB would actually be using an older version of Berkeley DB)
- Install Singularity on this host. Locate the centos.def file from the
example/directory. Edit to your heart’s desire (eg change OSVersion).
- Create the image, bootstrap, and run:
sudo singularity create /tmp/centos.img
sudo singularity bootstrap /tmp/centos.img centos.def
/tmp/centos.img to the host where you want to run the container (e.g. the Ubuntu host).
On the Ubuntu host, you can execute the CentOS container:
singularity shell centos.img
If further update is desired on this image, update the
centos.def as desired, then run:
singularity bootstrap centos.img centos.def
At this stage, the bootstrap works because the container already has the minimum requirements to run
yum from its own content. There isn’t a need to install
yum on the Ubuntu host.
Bootstrap a Docker Container (Option 4)
Instead of building your own seed CentOS image, the docker image imported using Option 4 can be used as well. Subsequent
singularity bootstrap on such .img file works.
Pursuing Option 1 or 2
You will need to find the binary for various versions of
db_load, and perhaps rename them to things like
db47_load, etc. You will then need to update the
build-yum.sh script that comes with the Singularity distribution, and add steps to convert the RPM DB files in
/var/lib/rpm to the desired version of Berkeley DB utilized by the target OS release.
For further details of the above steps, refer to this thread on the mailing list, and look for postings by Tru Huynh.
Miscellaneous troubleshooting notes
- The RPM command is NOT needed on the host to carry out the Singularity bootstrap process
- Initial bootstrap from an empty image needs
yum, but after a basic image with
yumin place, the
yuminstallation from inside the container is called.
- The RPM containing
db*_loadare different in different OS. Here is a helpful list:
OS rpm path to db*_load CentOS-6 db4-utils-4.7.25-20.el6_7.x86_64 /usr/bin/db_load CentOS-6 compat-db43-4.3.29-15.el6.x86_64 /usr/bin/db42_load CentOS-6 compat-db42-4.2.52-15.el6.x86_64 /usr/bin/db43_load CentOS-7 libdb-utils-5.3.21-19.el7.x86_64 /usr/bin/db_load
- Unfortunately the
filecommand provided by coreutils cannot give accurate version details of Berkeley DB used by the RPM database.
file /var/lib/rpm/Packagesreturns “version 9” in both CentOS 6 and 7.
- Unfortunately the
db_dumpcommand provided by
db4-utilsdoesn’t help either.
db_dump -p /var/lib/rpm/Packages | head -1always returns “VERSION=3”, for RPM DB found natively in RHEL-6 and 7 hosts.
Be careful with yum release
If building CentOS image from an Ubuntu host, one can seemingly use
yum --releasever=6 to get
yum to work and get a container to build. This kind of works, but some packages may be installed twice while others may not be consistent, since
yum is not able to properly query the RPM database created in the first stage of the bootstrap process. This approach is NOT recommended for any long-lived container images.