The client binaries are about 7 MB in size, or about 4 MB when debug symbols are stripped (using the
strip tool). This includes all dependencies for the client, such as the http, TLS, and JSON libraries.
To support atomic rootfs rollback, Mender integrates with the bootloader of the device. Currently Mender supports GRUB and U-Boot. The bootloader installation, features and requirements vary depending on the target OS in use. Please see the Yocto bootloader support or Debian bootloader support for more information.
While Mender itself does not have any specific kernel requirements beyond what a normal Linux kernel provides, it relies on systemd, which does have one such requirement: The
CONFIG_FHANDLE feature must be enabled in the kernel. The symptom if this feature is unavailable is that systemd hangs during boot looking for device files.
In order to support robust rollback, Mender requires the device to have a certain partition layout. At least four different partitions are needed:
One of the rootfs and kernel partitions will be marked as the active partition, from which the kernel and rootfs will be booted. The other, called the inactive partition, will be used by the update mechanism to write the updated image. After an update their roles are swapped.
The persistent data partition stores data that needs to be preserved through an update.
A sample partition layout is shown below:
Certificate verification requires the device clock to be running correctly at all times. Make sure to either have a reliable clock or use network time synchronization. Note that the default setup of systemd will use network time synchronization to maintain the clock in a running system. This may take a few minutes to stabilize on system boot so it is possible to have a few connection rejections from the server until this process is complete and the time is correct. Please see certificate troubleshooting for more information about the symptoms of this issue.
If your device does not have an active internet connection, then systemd will be unable to configure the system time as it will be unable to connect to the network time servers. In this case you will need to arrange other methods to set a proper system time. Many standard Linux features can be used for this. If your system includes a real-time clock chip, that will maintain the time across power down situations and the network connectivity needs of systemd will only be relevant on the system boots before the RTC is properly initialized.
Before the time is set properly, either by systemd or the RTC, the time will default to the Unix Epoch. Note that the Mender client connections will be rejected by the server until this situation is resolved.
Mender has official support for the Yocto build system and binary OS images based on the Debian family. It is possible to adapt to other build systems. Please see this blog post for an example (note that some of Mender's needs may have changed since the blog post was made).