openstack-manuals/doc/ops-guide/source/ops_maintenance_compute.rst

Compute Node Failures and Maintenance

Sometimes a compute node either crashes unexpectedly or requires a reboot for maintenance reasons.

Planned Maintenance

If you need to reboot a compute node due to planned maintenance, such as a software or hardware upgrade, perform the following steps:

1. Disable scheduling of new VMs to the node, optionally providing a reason comment:

   # nova service-disable --reason maintenance c01.example.com nova-compute

2. Verify that all hosted instances have been moved off the node:

   • If your cloud is using a shared storage:

     1. Get a list of instances that need to be moved:

        # nova list --host c01.example.com --all-tenants

     2. Migrate all instances one by one:

        # nova live-migration <uuid> c02.example.com

   • If your cloud is not using a shared storage, run:

     # nova live-migration --block-migrate <uuid> c02.example.com

3. Stop the nova-compute service:

   # stop nova-compute

   If you use a configuration-management system, such as Puppet, that ensures the nova-compute service is always running, you can temporarily move the init files:

   # mkdir /root/tmp
   # mv /etc/init/nova-compute.conf /root/tmp
   # mv /etc/init.d/nova-compute /root/tmp

4. Shut down your compute node, perform the maintenance, and turn the node back on.

5. Start the nova-compute service:

   # start nova-compute

   You can re-enable the nova-compute service by undoing the commands:

   # mv /root/tmp/nova-compute.conf /etc/init
   # mv /root/tmp/nova-compute /etc/init.d/

6. Enable scheduling of VMs to the node:

   # nova service-enable c01.example.com nova-compute

7. Optionally, migrate the instances back to their original compute node.

After a Compute Node Reboots

When you reboot a compute node, first verify that it booted successfully. This includes ensuring that the nova-compute service is running:

# ps aux | grep nova-compute
# status nova-compute

Also ensure that it has successfully connected to the AMQP server:

# grep AMQP /var/log/nova/nova-compute
2013-02-26 09:51:31 12427 INFO nova.openstack.common.rpc.common [-] Connected to AMQP server on 199.116.232.36:5672

After the compute node is successfully running, you must deal with the instances that are hosted on that compute node because none of them are running. Depending on your SLA with your users or customers, you might have to start each instance and ensure that they start correctly.

Instances

You can create a list of instances that are hosted on the compute node by performing the following command:

# nova list --host c01.example.com --all-tenants

After you have the list, you can use the nova command to start each instance:

# nova reboot <uuid>

Note

Any time an instance shuts down unexpectedly, it might have problems on boot. For example, the instance might require an fsck on the root partition. If this happens, the user can use the dashboard VNC console to fix this.

If an instance does not boot, meaning virsh list never shows the instance as even attempting to boot, do the following on the compute node:

# tail -f /var/log/nova/nova-compute.log

Try executing the nova reboot command again. You should see an error message about why the instance was not able to boot

In most cases, the error is the result of something in libvirt's XML file (/etc/libvirt/qemu/instance-xxxxxxxx.xml) that no longer exists. You can enforce re-creation of the XML file as well as rebooting the instance by running the following command:

# nova reboot --hard <uuid>

Inspecting and Recovering Data from Failed Instances

In some scenarios, instances are running but are inaccessible through SSH and do not respond to any command. The VNC console could be displaying a boot failure or kernel panic error messages. This could be an indication of file system corruption on the VM itself. If you need to recover files or inspect the content of the instance, qemu-nbd can be used to mount the disk.

Warning

If you access or view the user's content and data, get approval first!

To access the instance's disk (/var/lib/nova/instances/instance-xxxxxx/disk), use the following steps:

1. Suspend the instance using the virsh command.
2. Connect the qemu-nbd device to the disk.
3. Mount the qemu-nbd device.
4. Unmount the device after inspecting.
5. Disconnect the qemu-nbd device.
6. Resume the instance.

If you do not follow last three steps, OpenStack Compute cannot manage the instance any longer. It fails to respond to any command issued by OpenStack Compute, and it is marked as shut down.

Once you mount the disk file, you should be able to access it and treat it as a collection of normal directories with files and a directory structure. However, we do not recommend that you edit or touch any files because this could change the access control lists (ACLs) <access control list> that are used to determine which accounts can perform what operations on files and directories. Changing ACLs can make the instance unbootable if it is not already.

1. Suspend the instance using the virsh command, taking note of the internal ID:

   # virsh list
   Id Name                 State
   ----------------------------------
    1 instance-00000981    running
    2 instance-000009f5    running
   30 instance-0000274a    running
   
   # virsh suspend 30
   Domain 30 suspended

2. Connect the qemu-nbd device to the disk:

   # cd /var/lib/nova/instances/instance-0000274a
   # ls -lh
   total 33M
   -rw-rw---- 1 libvirt-qemu kvm  6.3K Oct 15 11:31 console.log
   -rw-r--r-- 1 libvirt-qemu kvm   33M Oct 15 22:06 disk
   -rw-r--r-- 1 libvirt-qemu kvm  384K Oct 15 22:06 disk.local
   -rw-rw-r-- 1 nova         nova 1.7K Oct 15 11:30 libvirt.xml
   # qemu-nbd -c /dev/nbd0 `pwd`/disk

3. Mount the qemu-nbd device.

   The qemu-nbd device tries to export the instance disk's different partitions as separate devices. For example, if vda is the disk and vda1 is the root partition, qemu-nbd exports the device as /dev/nbd0 and /dev/nbd0p1, respectively:

   # mount /dev/nbd0p1 /mnt/

   You can now access the contents of /mnt, which correspond to the first partition of the instance's disk.

   To examine the secondary or ephemeral disk, use an alternate mount point if you want both primary and secondary drives mounted at the same time:

   # umount /mnt
   # qemu-nbd -c /dev/nbd1 `pwd`/disk.local
   # mount /dev/nbd1 /mnt/
   # ls -lh /mnt/
   total 76K
   lrwxrwxrwx.  1 root root    7 Oct 15 00:44 bin -> usr/bin
   dr-xr-xr-x.  4 root root 4.0K Oct 15 01:07 boot
   drwxr-xr-x.  2 root root 4.0K Oct 15 00:42 dev
   drwxr-xr-x. 70 root root 4.0K Oct 15 11:31 etc
   drwxr-xr-x.  3 root root 4.0K Oct 15 01:07 home
   lrwxrwxrwx.  1 root root    7 Oct 15 00:44 lib -> usr/lib
   lrwxrwxrwx.  1 root root    9 Oct 15 00:44 lib64 -> usr/lib64
   drwx------.  2 root root  16K Oct 15 00:42 lost+found
   drwxr-xr-x.  2 root root 4.0K Feb  3  2012 media
   drwxr-xr-x.  2 root root 4.0K Feb  3  2012 mnt
   drwxr-xr-x.  2 root root 4.0K Feb  3  2012 opt
   drwxr-xr-x.  2 root root 4.0K Oct 15 00:42 proc
   dr-xr-x---.  3 root root 4.0K Oct 15 21:56 root
   drwxr-xr-x. 14 root root 4.0K Oct 15 01:07 run
   lrwxrwxrwx.  1 root root    8 Oct 15 00:44 sbin -> usr/sbin
   drwxr-xr-x.  2 root root 4.0K Feb  3  2012 srv
   drwxr-xr-x.  2 root root 4.0K Oct 15 00:42 sys
   drwxrwxrwt.  9 root root 4.0K Oct 15 16:29 tmp
   drwxr-xr-x. 13 root root 4.0K Oct 15 00:44 usr
   drwxr-xr-x. 17 root root 4.0K Oct 15 00:44 var

4. Once you have completed the inspection, unmount the mount point and release the qemu-nbd device:

   # umount /mnt
   # qemu-nbd -d /dev/nbd0
   /dev/nbd0 disconnected

5. Resume the instance using virsh:

   # virsh list
   Id Name                 State
   ----------------------------------
    1 instance-00000981    running
    2 instance-000009f5    running
   30 instance-0000274a    paused
   
   # virsh resume 30
   Domain 30 resumed

Volumes

If the affected instances also had attached volumes, first generate a list of instance and volume UUIDs:

mysql> select nova.instances.uuid as instance_uuid,
       cinder.volumes.id as volume_uuid, cinder.volumes.status,
       cinder.volumes.attach_status, cinder.volumes.mountpoint,
       cinder.volumes.display_name from cinder.volumes
       inner join nova.instances on cinder.volumes.instance_uuid=nova.instances.uuid
       where nova.instances.host = 'c01.example.com';

You should see a result similar to the following:

+--------------+------------+-------+--------------+-----------+--------------+
|instance_uuid |volume_uuid |status |attach_status |mountpoint | display_name |
+--------------+------------+-------+--------------+-----------+--------------+
|9b969a05      |1f0fbf36    |in-use |attached      |/dev/vdc   | test         |
+--------------+------------+-------+--------------+-----------+--------------+
1 row in set (0.00 sec)

Next, manually detach and reattach the volumes, where X is the proper mount point:

# nova volume-detach <instance_uuid> <volume_uuid>
# nova volume-attach <instance_uuid> <volume_uuid> /dev/vdX

Be sure that the instance has successfully booted and is at a login screen before doing the above.

Total Compute Node Failure

Compute nodes can fail the same way a cloud controller can fail. A motherboard failure or some other type of hardware failure can cause an entire compute node to go offline. When this happens, all instances running on that compute node will not be available. Just like with a cloud controller failure, if your infrastructure monitoring does not detect a failed compute node, your users will notify you because of their lost instances.

If a compute node fails and won't be fixed for a few hours (or at all), you can relaunch all instances that are hosted on the failed node if you use shared storage for /var/lib/nova/instances.

To do this, generate a list of instance UUIDs that are hosted on the failed node by running the following query on the nova database:

mysql> select uuid from instances
       where host = 'c01.example.com' and deleted = 0;

Next, update the nova database to indicate that all instances that used to be hosted on c01.example.com are now hosted on c02.example.com:

mysql> update instances set host = 'c02.example.com'
       where host = 'c01.example.com' and deleted = 0;

If you're using the Networking service ML2 plug-in, update the Networking service database to indicate that all ports that used to be hosted on c01.example.com are now hosted on c02.example.com:

mysql> update ml2_port_bindings set host = 'c02.example.com'
       where host = 'c01.example.com';
mysql> update ml2_port_binding_levels set host = 'c02.example.com'
       where host = 'c01.example.com';

After that, use the nova command to reboot all instances that were on c01.example.com while regenerating their XML files at the same time:

# nova reboot --hard <uuid>

Finally, reattach volumes using the same method described in the section volumes.

/var/lib/nova/instances

It's worth mentioning this directory in the context of failed compute nodes. This directory contains the libvirt KVM file-based disk images for the instances that are hosted on that compute node. If you are not running your cloud in a shared storage environment, this directory is unique across all compute nodes.

/var/lib/nova/instances contains two types of directories.

The first is the _base directory. This contains all the cached base images from glance for each unique image that has been launched on that compute node. Files ending in _20 (or a different number) are the ephemeral base images.

The other directories are titled instance-xxxxxxxx. These directories correspond to instances running on that compute node. The files inside are related to one of the files in the _base directory. They're essentially differential-based files containing only the changes made from the original _base directory.

All files and directories in /var/lib/nova/instances are uniquely named. The files in _base are uniquely titled for the glance image that they are based on, and the directory names instance-xxxxxxxx are uniquely titled for that particular instance. For example, if you copy all data from /var/lib/nova/instances on one compute node to another, you do not overwrite any files or cause any damage to images that have the same unique name, because they are essentially the same file.

Although this method is not documented or supported, you can use it when your compute node is permanently offline but you have instances locally stored on it.