tripleo-image-elements/elements/use-ephemeral

Provides tools to aid in relocating state to /mnt/state

In a cloud instance /mnt/state is on the cloud ephemeral device when one exists, or the root when one doesn't.

The pattern for having state on /mnt/state is:

• The service/daemon can make its own state files on startup (as /mnt is always empty on first-boot).

• Alternatively os-refresh-config can be used to populate any initial state so that the daemon/service works (so until the first os-refresh-config run that daemon/service will be broken).

• If the state needs migrating (e.g. PostgreSQL 9.1 to 9.2) then a migration script should be added to os-refresh-config's migrations.d tree to perform that migration that state (if the migration can be done after the service starts) or to configure.d if the service cannot start without the migration being done).

• state should live in /mnt/state/original-path e.g. /mnt/state/etc/ssh/ssh_host_dsa_key.

• where there are hardcoded specific paths, we leave symlinks in that path pointing into /mnt/state - e.g. /etc/ssh/ss_host_dsa_key will be a symlink to /mnt/state/etc/ssh/ssh_host_dsa_key.

To factor out common code elements can invoke register-state-path during install.d to request that a particular path be registered as a stateful path.

• If there is content at the path at registration time, it will be moved to /var/lib/use-ephemeral/original-path.

• If --leave-symlink is passed, a symlink will be created at that path pointing to /mnt/state/original-path.

• Stateful paths are listed in /var/lib/use-ephemeral/stateful-paths, one path per line - e.g. /etc/ssh/ssh_host_dsa_key.

Once registered:

• During pre-configure.d the parent directories leading up to the path will be asserted on startup.

• If there is a content at /var/lib/use-ephemeral/original-path during pre-configure.d, and the new path does not exist in /mnt/state then an rsync -a will be made into /mnt/state/original-path reinstating a pristine copy of what was preserved. This is only done when the path does not exist to avoid corrupting evolved or migrated state.