Preseed performance
When Ubuntu Core boots for the first time, a seeding process installs an initial set of snaps and runs their respective hooks. Preseeding speeds up the seeding process by performing as many of these administrative tasks as possible in advance when an image is being created.
The resultant image is called a preseeded image, and a preseeded image is particularly effective on devices with limited network bandwidth.
With a preseeded image, the snap installation and update process is performed as part of the image creation process, before device deployment. This means a device is ready to go from the first boot, rather than requiring the further snap installation and updates that may be required with a non-preseeded image.
For details on how to create a preseeded image, see Optimise boot speed.
Measuring boot performance
The time it takes for a new device to be ready can be measured with the ubuntu_core.bootchart kernel option. When enabled, a performance profile is built for each device boot, allowing you to compare how a non-preseeded image performs against a preseeded image.
The following are real examples of bootchart performance profiles, generated for the same deployment with and without preseeding and initialised over cell data.
Without preseeding
Without preseeding, a device will need to reboot after the base and system snaps have been updated, and this has a significant impact on the length of time a device takes to become available.H
We can see this in our test deployment, with the generation of two bootcharts for each boot before the system becomes accessible. The total time it takes for those two boots to complete is 71.7 seconds, not including the early boot stages before bootchart starts.
First boot: 22.8 seconds
Second boot: 48.9 seconds
Total time before the system becomes accessible: 71.7 seconds
With preseeding
Our preseeded system did not need a reboot, and neither did it require any further updates or snap packages. This resulted in the system becoming accessible much faster, and as soon as the first boot had completed:
First boot: 11.4 seconds
Total time before the system becomes accessible: 11.4 seconds
Bootcharts
All three bootchart excerpts below show:
- ping read from storage utilisation
- red write to storage utilisation
- blue CPU utilisation
- yellow CPU wait time
Without preseeding
The following shows two separate boots, the first for 22.8 seconds and the second lasting 48.9 seconds.
With preseeding
The following shows the only boot required for a preseeded image, lasting 11.4 seconds.
