Configurable /mnt/state
This spec covers making the top level directory where persistent and stateful data is written to configurable. Currently this top level path is hardcoded to /mnt/state in our os-apply-config templates, os-refresh-config scripts, and other element scripts. Change-Id: Ifdaf31a97a76d69d89c0b5f48714cedfb3cf7b06
This commit is contained in:
James Slagle
Alexis Lee
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specs/juno/tripleo-juno-configurable-mnt-state.rst
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specs/juno/tripleo-juno-configurable-mnt-state.rst
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This work is licensed under a Creative Commons Attribution 3.0 Unported
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License.
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http://creativecommons.org/licenses/by/3.0/legalcode
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=======================================================
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Configurable directory for persistent and stateful data
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=======================================================
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https://blueprints.launchpad.net/tripleo/+spec/tripleo-juno-configurable-mnt-state
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Make the hardcoded /mnt/state path for stateful data be configurable.
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Problem Description
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===================
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1. A hard coded directory of /mnt/state for persistent data is incompatible
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with Red Hat based distros available mechanism for a stateful data path. Red
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Hat based distros, such as Fedora, RHEL, and CentOS, have a feature that uses
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bind mounts for mounting paths onto a stateful data partition and does not
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require manually reconfiguring software to use /mnt/state.
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2. Distros that use SELinux have pre-existing policy that allows access to
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specific paths. Reconfiguring these paths to be under /mnt/state, results
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in SELinux denials for existing services, requiring additional policy to be
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written and maintained.
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3. Some operators and administrators find the reconfiguring of many services to
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not use well known default values for filesystem paths to be disruptive and
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inconsistent. They do not expect these changes when using a distro that they've
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come to learn and anticipate certain configurations. These types of changes
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also require documentation changes to existing documents and processes.
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Proposed Change
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===============
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Deployers will be able to choose a configurable path instead of the hardcoded
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value of /mnt/state for the stateful path.
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A new element, stateful-path will be added that defines the value for the
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stateful path. The default will be /mnt/state.
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There are 3 areas that need to respect the configurable path:
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os-apply-config template generation
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The stateful-path element will set the stateful path value by installing a
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JSON file to a well known location for os-collect-config to use as a local
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data source. This will require a new local data source collector to be added
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to os-collect-config (See `Dependencies`_).
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The JSON file's contents will be based on $STATEFUL_PATH, e.g.:
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{'stateful-path': '/mnt/state'}
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File templates (files under os-apply-config in an element) will then be
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updated to replace the hard coded /mnt/state with {{stateful-path}}.
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Currently, there is a mix of root locations of the os-apply-config templates.
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Most are written under /, although some are written under /mnt/state. The
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/mnt/state is hard coded in the directory tree under os-apply-config in these
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elements, so this will be removed to have the templates just written under /.
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Symlinks could instead be used in these elements to setup the correct paths.
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Support can also be added to os-apply-config's control file mechanism to
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indicate these files should be written under the stateful path. An example
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patch that does this is at: https://review.openstack.org/#/c/113651/
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os-refresh-config scripts run at boot time
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In order to make the stateful path configurable, all of the hard coded
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references to /mnt/state in os-refresh-config scripts will be replaced with an
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environment variable, $STATEFUL_PATH.
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The stateful-path element will provide an environment.d script for
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os-refresh-config that reads the value from os-apply-config:
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export STATEFUL_PATH=$(os-apply-config --key stateful-path --type raw)
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Hook scripts run at image build time
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The stateful-path element will provide an environment.d script for use at
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image build time:
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export STATEFUL_PATH=${STATEFUL_PATH:-"/mnt/state"}
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The use-ephemeral element will depend on the stateful-path element, effectively
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making the default stateful path remain /mnt/state.
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The stateful path can be reconfigured by defining $STATEFUL_PATH either A) in
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the environment before an image build; or B) in an element with an
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environment.d script which runs earlier than the stateful-path environment.d
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script.
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Alternatives
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------------
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None come to mind, the point of this spec is to enable an alternative to what's
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already existing. There may be additional alternatives out there other folks
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may wish to add support for.
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Security Impact
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---------------
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None
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Other End User Impact
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---------------------
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End users using elements that change the stateful path location from /mnt/state
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to something else will see this change reflected in configuration files and in
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the directories used for persistent and stateful data. They will have to know
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how the stateful path is configured and accessed.
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Different TripleO installs would appear different if used with elements that
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configured the stateful path differently.
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This also adds some complexity when reading TripleO code, because instead of
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there being an explicit path, there would instead be a reference to a
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configurable value.
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Performance Impact
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------------------
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There will be additional logic in os-refresh-config to determine and set the
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stateful path, and an additional local collector that os-collect-config would
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use. However, these are negligible in terms of negatively impacting
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performance.
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Other Deployer Impact
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---------------------
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Deployers will be able to choose different elements that may reconfigure the
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stateful path or change the value for $STATEFUL_PATH. The default will remain
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unchanged however.
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Deployers would have to know what the stateful path is, and if it's different
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across their environment, this could be confusing. However, this seems unlikely
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as deployers are likely to be standardizing on one set of common elements,
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distro, etc.
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In the future, if TripleO CI and CD clouds that are based on Red Hat distros
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make use of this feature to enable Red Hat read only root support, then these
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clouds would be configured differently from clouds that are configured to use
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/mnt/state. As a team, the tripleo-cd-admins will have to know which
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configuration has been used.
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Developer Impact
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----------------
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1. Developers need to use the $STATEFUL_PATH and {{stateful-path}}
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substitutions when they intend to refer to the stateful path.
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2. Code that needs to know the stateful path will need access to the variable
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defining the path, it won't be able to assume the path is /mnt/state. A call to
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os-apply-config to query the key defining the path could be done to get
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the value, as long as os-collect-config has already run at least once.
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Implementation
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==============
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Assignee(s)
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-----------
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Primary assignee:
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james-slagle
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Work Items
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----------
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tripleo-incubator
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^^^^^^^^^^^^^^^^^
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* Update troubleshooting docs to mention that /mnt/state is a configurable
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path, and could be different in local environments.
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tripleo-image-elements
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^^^^^^^^^^^^^^^^^^^^^^
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* Add a new stateful-path element that configures stateful-path and $STATEFUL_PATH
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to /mnt/state
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* Update os-apply-config templates to replace /mnt/state with {{stateful-path}}
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* Update os-refresh-config scripts to replace /mnt/state with $STATEFUL_PATH
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* Update all elements that have os-apply-config template files under /mnt/state
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to just be under /.
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* update os-apply-config element to call os-apply-config with a --root
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$STATEFUL_PATH option
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* update elements that have paths to os-apply-config generated files (such
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as /etc/nova/nova.conf) to refer to those paths as
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$STATEFUL_PATH/path/to/file.
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* make use-ephemeral element depend on stateful-path element
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Dependencies
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============
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1. os-collect-config will need a new feature to read from a local data source
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directory that elements can install JSON files into, such as a source.d. There
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will be a new spec filed on this feature.
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https://review.openstack.org/#/c/100965/
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2. os-apply-config will need an option in its control file to support
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generating templates under the configurable stateful path. There is a patch
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here: https://review.openstack.org/#/c/113651/
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Testing
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=======
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There is currently no testing that all stateful and persistent data is actually
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written to a stateful partition.
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We should add tempest tests that directly exercise the preserve_ephemeral
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option, and have tests that check that all stateful data has been preserved
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across a "nova rebuild". Tempest seems like a reasonable place to add these
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tests since preserve_ephemeral is a Nova OpenStack feature. Plus, once TripleO
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CI is running tempest against the deployed OverCloud, we will be testing this
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feature.
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We should also test in TripleO CI that state is preserved across a rebuild by
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adding stateful data before a rebuild and verifying it is still present after a
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rebuild.
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Documentation Impact
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====================
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We will document the new stateful-path element.
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TripleO documentation will need to mention the potential difference in
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configuration files and the location of persistent data if a value other than
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/mnt/state is used.
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References
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==========
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os-collect-config local datasource collector spec:
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* https://review.openstack.org/100965
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Red Hat style stateful partition support this will enable:
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* https://git.fedorahosted.org/cgit/initscripts.git/tree/systemd/fedora-readonly
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* https://git.fedorahosted.org/cgit/initscripts.git/tree/sysconfig/readonly-root
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* https://git.fedorahosted.org/cgit/initscripts.git/tree/statetab
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* https://git.fedorahosted.org/cgit/initscripts.git/tree/rwtab
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