To maintain build history, it's important to optimize it and prevent it from growing over time. Periodic backups of build history and clean-ups of older data are important to keep the build history repository at a manageable size.

Once the buildhistory directory has been backed up, the following process will trim it and keep only the most recent history:

1. Copy your repository to a temporary RAM filesystem (tmpfs) to speed things up. Check the output of the df -h command to see which directories are tmpfs filesystems and how much space they have available, and use one. For example, in Ubuntu 16.04, the /run/ directory is available.

2. Copy build history to the /run directory as follows:

$ sudo mkdir /run/workspace
$ sudo chown ${USER} /run/workspace/
$ cp -r /opt/yocto/fsl-community-bsp/wandboard/buildhistory/ /run/workspace/
$ cd /run/workspace/buildhistory/  

3. Add a graft point for a commit 1 month ago with no parents:

$ git rev-parse "HEAD@{1 month ago}" > .git/info/grafts  

4. Make the graft point permanent:

$ git filter-branch

5. Clone a new repository to clean up the remaining Git objects:

$ git clone file://${tmpfs}/buildhistory buildhistory.new

6. Replace the old buildhistory directory with the new cleaned one:

$ rm -rf buildhistory
$ mv buildhistory.new /opt/yocto/fsl-community-bsp/wandboard/buildhistory/

7. And finally, remove the workspace:

$ rm -rf /run/workspace/