openstack-manuals/doc/admin-guide-cloud/source/compute-node-down.rst

.. _section_nova-compute-node-down:

==================================
Recover from a failed compute node
==================================

If you deploy Compute with a shared file system, you can use several methods
to quickly recover from a node failure. This section discusses manual
recovery.

Evacuate instances
~~~~~~~~~~~~~~~~~~

If a hardware malfunction or other error causes the cloud compute node to
fail, you can use the :command:`nova evacuate` command to evacuate instances.
See the `Admin User Guide <http://docs.openstack.org/user-guide-admin/cli_nova_evacuate.html>`__.

.. _nova-compute-node-down-manual-recovery:

Manual recovery
~~~~~~~~~~~~~~~
To manually recover a failed compute node:

#. Identify the VMs on the affected hosts by using a combination of
   the :command:`nova list` and :command:`nova show` commands or the
   :command:`euca-describe-instances` command.

   For example, this command displays information about the i-000015b9
   instance that runs on the np-rcc54 node:

   ..  code-block:: console

       $ euca-describe-instances
       i-000015b9 at3-ui02 running nectarkey (376, np-rcc54) 0 m1.xxlarge 2012-06-19T00:48:11.000Z 115.146.93.60

#. Query the Compute database for the status of the host. This example
   converts an EC2 API instance ID to an OpenStack ID. If you use the
   :command:`nova` commands, you can substitute the ID directly. This example
   output is truncated:

   .. code-block:: mysql

      mysql> SELECT * FROM instances WHERE id = CONV('15b9', 16, 10) \G;
      *************************** 1. row ***************************
      created_at: 2012-06-19 00:48:11
      updated_at: 2012-07-03 00:35:11
      deleted_at: NULL
      ...
      id: 5561
      ...
      power_state: 5
      vm_state: shutoff
      ...
      hostname: at3-ui02
      host: np-rcc54
      ...
      uuid: 3f57699a-e773-4650-a443-b4b37eed5a06
      ...
      task_state: NULL
      ...

   .. note::

      Find the credentials for your database in ``/etc/nova.conf`` file.

#. Decide to which compute host to move the affected VM. Run this database
   command to move the VM to that host:

   .. code-block:: mysql

      mysql> UPDATE instances SET host = 'np-rcc46' WHERE uuid = '3f57699a-e773-4650-a443-b4b37eed5a06';

#. If you use a hypervisor that relies on libvirt, such as KVM, update the
   ``libvirt.xml`` file in ``/var/lib/nova/instances/[instance ID]`` with
   these changes:

   -  Change the ``DHCPSERVER`` value to the host IP address of the new
      compute host.

   -  Update the VNC IP to ``0.0.0.0``.

#. Reboot the VM:

   .. code-block:: console

      $ nova reboot 3f57699a-e773-4650-a443-b4b37eed5a06

Typically, the database update and :command:`nova reboot` command recover a VM
from a failed host. However, if problems persist, try one of these actions:

* Use :command:`virsh` to recreate the network filter configuration.
* Restart Compute services.
* Update the ``vm_state`` and ``power_state`` fields in the Compute database.

.. _section_nova-uid-mismatch:

Recover from a UID/GID mismatch
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Sometimes when you run Compute with a shared file system or an automated
configuration tool, files on your compute node might use the wrong UID or GID.
This UID or GID mismatch can prevent you from running live migrations or
starting virtual machines.

This procedure runs on ``nova-compute`` hosts, based on the KVM hypervisor:

#. Set the nova UID to the same number in ``/etc/passwd`` on all hosts. For
   example, set the UID to ``112``.

   .. note::

      Choose UIDs or GIDs that are not in use for other users or groups.

#. Set the ``libvirt-qemu`` UID to the same number in the ``/etc/passwd`` file
   on all hosts. For example, set the UID to ``119``.

#. Set the ``nova`` group to the same number in the ``/etc/group`` file on all
   hosts. For example, set the group to ``120``.

#. Set the ``libvirtd`` group to the same number in the ``/etc/group`` file on
   all hosts. For example, set the group to ``119``.

#. Stop the services on the compute node.

#. Change all files that the nova user or group owns. For example:

   .. code-block:: console

      # find / -uid 108 -exec chown nova {} \;
      # note the 108 here is the old nova UID before the change
      # find / -gid 120 -exec chgrp nova {} \;

#. Repeat all steps for the ``libvirt-qemu`` files, if required.

#. Restart the services.

#. To verify that all files use the correct IDs, run the :command:`find`
   command.

.. _section_nova-disaster-recovery-process:

Recover cloud after disaster
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This section describes how to manage your cloud after a disaster and back up
persistent storage volumes. Backups are mandatory, even outside of disaster
scenarios.

For a definition of a disaster recovery plan (DRP), see
`http://en.wikipedia.org/wiki/Disaster\_Recovery\_Plan <http://en.wikipedia.org/wiki/Disaster_Recovery_Plan>`_.

A disk crash, network loss, or power failure can affect several components in
your cloud architecture. The worst disaster for a cloud is a power loss. A
power loss affects these components:

-  A cloud controller (``nova-api``, ``nova-objectstore``, ``nova-network``)

-  A compute node (``nova-compute``)

-  A storage area network (SAN) used by OpenStack Block Storage
   (``cinder-volumes``)

Before a power loss:

-  Create an active iSCSI session from the SAN to the cloud controller
   (used for the ``cinder-volumes`` LVM's VG).

-  Create an active iSCSI session from the cloud controller to the compute
   node (managed by ``cinder-volume``).

-  Create an iSCSI session for every volume (so 14 EBS volumes requires 14
   iSCSI sessions).

-  Create ``iptables`` or ``ebtables`` rules from the cloud controller to the
   compute node. This allows access from the cloud controller to the
   running instance.

-  Save the current state of the database, the current state of the running
   instances, and the attached volumes (mount point, volume ID, volume
   status, etc), at least from the cloud controller to the compute node.

After power resumes and all hardware components restart:

-  The iSCSI session from the SAN to the cloud no longer exists.

-  The iSCSI session from the cloud controller to the compute node no
   longer exists.

-  nova-network reapplies configurations on boot and, as a result, recreates
   the iptables and ebtables from the cloud controller to the compute node.

-  Instances stop running.

   Instances are not lost because neither ``destroy`` nor ``terminate`` ran.
   The files for the instances remain on the compute node.

-  The database does not update.

**Begin recovery**

.. warning::

   Do not add any steps or change the order of steps in this procedure.

#. Check the current relationship between the volume and its instance, so
   that you can recreate the attachment.

   Use the :command:`nova volume-list` command to get this information. Note
   that the :command:`nova` client can get volume information from OpenStack Block
   Storage.

#. Update the database to clean the stalled state. Do this for every
   volume by using these queries:

   .. code-block:: mysql

      mysql> use cinder;
      mysql> update volumes set mountpoint=NULL;
      mysql> update volumes set status="available" where status <>"error_deleting";
      mysql> update volumes set attach_status="detached";
      mysql> update volumes set instance_id=0;

   Use :command:`nova volume-list` command to list all volumes.

#. Restart the instances by using the :command:`nova reboot INSTANCE` command.

   .. important::

      Some instances completely reboot and become reachable, while some might
      stop at the plymouth stage. This is expected behavior. DO NOT reboot a
      second time.

      Instance state at this stage depends on whether you added an
      `/etc/fstab` entry for that volume. Images built with the cloud-init
      package remain in a ``pending`` state, while others skip the missing
      volume and start. You perform this step to ask Compute to reboot every
      instance so that the stored state is preserved. It does not matter if
      not all instances come up successfully. For more information about
      cloud-init, see
      `help.ubuntu.com/community/CloudInit/ <https://help.ubuntu.com/community/CloudInit/>`__.

#. If required, run the :command:`nova volume-attach` command to reattach the
   volumes to their respective instances. This example uses a file of listed
   volumes to reattach them:

   .. code-block:: bash

      #!/bin/bash

      while read line; do
          volume=`echo $line | $CUT -f 1 -d " "`
          instance=`echo $line | $CUT -f 2 -d " "`
          mount_point=`echo $line | $CUT -f 3 -d " "`
              echo "ATTACHING VOLUME FOR INSTANCE - $instance"
          nova volume-attach $instance $volume $mount_point
          sleep 2
      done < $volumes_tmp_file

   Instances that were stopped at the plymouth stage now automatically
   continue booting and start normally. Instances that previously started
   successfully can now see the volume.

#. Log in to the instances with SSH and reboot them.

   If some services depend on the volume or if a volume has an entry in fstab,
   you can now restart the instance. Restart directly from the instance itself
   and not through :command:`nova`:

   .. code-block:: console

      # shutdown -r now

   When you plan for and complete a disaster recovery, follow these tips:

-  Use the ``errors=remount`` option in the ``fstab`` file to prevent
   data corruption.

   In the event of an I/O error, this option prevents writes to the disk. Add
   this configuration option into the cinder-volume server that performs the
   iSCSI connection to the SAN and into the instances' ``fstab`` files.

-  Do not add the entry for the SAN's disks to the cinder-volume's
   ``fstab`` file.

   Some systems hang on that step, which means you could lose access to
   your cloud-controller. To re-run the session manually, run this
   command before performing the mount:

   .. code-block:: console

      # iscsiadm -m discovery -t st -p $SAN_IP $ iscsiadm -m node --target-name $IQN -p $SAN_IP -l

-  On your instances, if you have the whole ``/home/`` directory on the
   disk, leave a user's directory with the user's bash files and the
   ``authorized_keys`` file instead of emptying the ``/home/`` directory
   and mapping the disk on it.

   This action enables you to connect to the instance without the volume
   attached, if you allow only connections through public keys.

To script the disaster recovery plan (DRP), use the
`https://github.com/Razique <https://github.com/Razique/BashStuff/blob/master/SYSTEMS/OpenStack/SCR_5006_V00_NUAC-OPENSTACK-DRP-OpenStack.sh>`_ bash script.

This script completes these steps:

#. Creates an array for instances and their attached volumes.

#. Updates the MySQL database.

#. Restarts all instances with euca2ools.

#. Reattaches the volumes.

#. Uses Compute credentials to make an SSH connection into every instance.

The script includes a ``test mode``, which enables you to perform the sequence
for only one instance.

To reproduce the power loss, connect to the compute node that runs that
instance and close the iSCSI session. Do not detach the volume by using the
:command:`nova volume-detach` command. You must manually close the iSCSI
session. This example closes an iSCSI session with the number ``15``:

.. code-block:: console

   # iscsiadm -m session -u -r 15

Do not forget the :option:`-r` option. Otherwise, all sessions close.