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

Recover from a failed compute node

If Compute is deployed with a shared file system, and a node fails, there are several methods to quickly recover from the failure. This section discusses manual recovery.

Evacuate instances

If a cloud compute node fails due to a hardware malfunction or another reason, you can evacuate instances using the nova evacuate command. See the Admin User Guide.

Manual recovery

Use this procedure to recover a failed compute node manually:

1. Identify the VMs on the affected hosts. To do this, you can use a combination of nova list and nova show or euca-describe-instances. For example, this command displays information about instance i-000015b9 that is running on node np-rcc54:

   $ 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

2. Query the Compute database to check the status of the host. This example converts an EC2 API instance ID into an OpenStack ID. If you use the nova commands, you can substitute the ID directly (the output in this example has been truncated):

   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

   The credentials for your database can be found in /etc/nova.conf.

3. Decide which compute host the affected VM should be moved to, and run this database command to move the VM to the new host:

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

4. If you are using a hypervisor that relies on libvirt (such as KVM), update the libvirt.xml file (found 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

5. Reboot the VM:

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

The database update and nova reboot command should be all that is required to recover a VM from a failed host. However, if you continue to have problems try recreating the network filter configuration using virsh, restarting the Compute services, or updating the vm_state and power_state in the Compute database.

Recover from a UID/GID mismatch

In some cases, files on your compute node can end up using the wrong UID or GID. This can happen when running OpenStack Compute, using a shared file system, or with an automated configuration tool. This can cause a number of problems, such as inability to perform live migrations, or start virtual machines.

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

1. Set the nova UID in /etc/passwd to the same number on all hosts (for example, 112).

   Note

   Make sure you choose UIDs or GIDs that are not in use for other users or groups.

2. Set the libvirt-qemu UID in /etc/passwd to the same number on all hosts (for example, 119).

3. Set the nova group in /etc/group file to the same number on all hosts (for example, 120).

4. Set the libvirtd group in /etc/group file to the same number on all hosts (for example, 119).

5. Stop the services on the compute node.

6. Change all the files owned by user or group nova. For example:

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

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

8. Restart the services.

9. Run the find command to verify that all files use the correct identifiers.

Recover cloud after disaster

This section covers procedures for managing your cloud after a disaster, and backing 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.

A disaster could happen to several components of your architecture (for example, a disk crash, network loss, or a power failure). In this example, the following components are configured:

• 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)

The worst disaster for a cloud is power loss, which applies to all three components. 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 is recovered and all hardware components have restarted:

• 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.

• The iptables and ebtables from the cloud controller to the compute node are recreated. This is because nova-network reapplies configurations on boot.

• Instances are no longer running.

  Note that instances will not be lost, because neither destroy nor terminate was invoked. The files for the instances will remain on the compute node.

• The database has not been updated.

Begin recovery

Warning

Do not add any extra steps to this procedure, or perform the steps out of order.

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

   This information can be found using the nova volume-list command. Note that the nova client also includes the ability to get volume information from OpenStack Block Storage.

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

   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 nova volume-list commands to list all volumes.

3. Restart the instances using the nova reboot INSTANCE command.

   Important

   Some instances will 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. This step is performed in order 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/.

4. Reattach the volumes to their respective instances, if required, using the nova volume-attach command. This example uses a file of listed volumes to reattach them:

   #!/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 will now automatically continue booting and start normally. Instances that previously started successfully will now be able to see the volume.

5. 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 should now be able to restart the instance. Restart directly from the instance itself, not through nova:

   # shutdown -r now

   When you are planning for and performing a disaster recovery, follow these tips:

• Use the errors=remount parameter in the fstab file to prevent data corruption.

  This parameter will cause the system to disable the ability to write to the disk if it detects an I/O error. This configuration option should be added into the cinder-volume server (the one which 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:

  # 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 allows you to connect to the instance even without the volume attached, if you allow only connections through public keys.

If you want to script the disaster recovery plan (DRP), a bash script is available from https://github.com/Razique which performs the following steps:

1. An array is created for instances and their attached volumes.
2. The MySQL database is updated.
3. All instances are restarted with euca2ools.
4. The volumes are reattached.
5. An SSH connection is performed into every instance using Compute credentials.

The script includes a test mode, which allows you to perform that whole sequence for only one instance.

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

# iscsiadm -m session -u -r 15

Do not forget the -r flag. Otherwise, you will close all sessions.