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docs: Add section about staged deployments

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I was explaining staged deployments to someone today and was looking for
a doc but we didn't have any. Fix that.
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Jonathan Lebon 2022-08-23 10:56:28 -04:00
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commit e075c51057
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docs/deployment.md
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@ -85,6 +85,26 @@ At the moment, OSTree only supports upgrading a single refspec.
However, in the future OSTree may support a syntax for composing
layers of trees, for example.
### Staged deployments
As mentioned above, when OSTree creates a new deployment, a 3-way merge is done
to update its `/etc`. Depending on the nature of the system, this can cause an
issue: if a user or program modifies the booted `/etc` *after* the pending
deployment is created but *before* rebooting, those modifications will be lost.
OSTree does not do a second `/etc` merge on reboot.
To counter this, OSTree supports staged deployments. In this flow, deployments
are created using e.g. `ostree admin upgrade --stage` on the CLI. The new
deployment is still created when the command is invoked, but the 3-way `/etc`
merge is delayed until the system is rebooted or shut down. Additionally,
updating the bootloader is also delayed. This is done by the
`ostree-finalize-staged.service` systemd unit.
The main disadvantage of this approach is that rebooting can take longer and the
failure mode can be confusing (the machine will reboot into the same
deployment). In systems where the workload is well-understood and not subject to
the `/etc` issue above, it may be better to not stage deployments.
### The system /boot
While OSTree parallel installs deployments cleanly inside the