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To support an easy golden image workflow, Mender supports creating a snapshot of the currently running system. In this workflow a "golden image" is maintained on a single device (or SD card) by making run-time modifications on the device, such as installing packages, changing configurations and updating the operating system. When this device has been tested and the environment should be replicated, a snapshot can be taken with Mender. This results in a file system image and Mender Artifact that can be deployed to the rest of the device fleet.
This feature is fully supported by the following filesystems: ext, XFS, JFS, btrfs, f2fs, and ReiserFS.
mender executable provides a command
mender snapshot dump for dumping a
frozen copy of the root filesystem to standard error. In this section, we go
through two common approaches for using the snapshot feature from the device:
dumping a snapshot to a remote host or to a storage device.
While a snapshot is in progress, all processes that writes to the root filesystem will be blocked for the duration of the snapshot process. Redirecting the output of the snapshot command to the same filesystem will freeze the system for a short duration before aborting.
ssh is available on your device, then it is possible to redirect the output
from the snapshot command to a remote host. Assuming a computer is reachable and
running the ssh daemon, running the following command will generate a snapshot
root-part.ext4 in the user's home directory on the remote machine:
USER="user" HOST="host-ip" mender snapshot dump | ssh $USER@$HOST /bin/sh -c 'cat > $HOME/root-part.ext4`
ssh is not available, you can attach a removable storage device (e.g.
USB stick) and redirect the output to a file on the device.
mount /dev/(...) /mnt mender snapshot dump > /mnt/root-part.ext4
Make sure there is enough available space on the storage device for the
entire root filesystem (e.g. comparing the output of
df -h / /mnt).
To help save storage space and bandwidth, a built-in
--compression option is
available. For the example above, a gzip-compressed version of the filesystem is
produced by passing gzip to the
mount /dev/(...) /mnt mender snapshot dump --compression gzip > /mnt/root-part.ext4.gz
Don't forget the
.gz extension in the target filename.
In this case, passing
root-part.ext4.gz) as the
file-parameter to mender-artifact
produces a deployment ready Mender Artifact:
mender-artifact write rootfs-image -f /mnt/root-part.ext4 \ -n artifact-name \ -o snapshot-release.1.0.mender \ -t device-type
Uploading this artifact to the mender server and creating a deployment ensures that all devices runs an identical rootfs version as the golden device.
There is support for creating a snapshot artifact directly from a workstation
with mender-artifact installed. This approach requires that the golden device
is reachable and has ssh and sudo installed.
mender-artifact accepts a
file URL with ssh schema and will automatically run the commands above
on the golden device. For example, the command:
USER="user" ADDR="device-ip:port" mender-artifact write rootfs-image -f ssh://$USER@$ADDR \ -n artifact-name \ -o snapshot-release.1.0.mender \ -t device-type
Yields the exact same artifact as above.