OpenStack volumes are persistent block-storage devices that may be attached and detached from instances, but they can be attached to only one instance at a time. Similar to an external hard drive, they do not provide shared storage in the way a network file system or object store does. It is left to the operating system in the instance to put a file system on the block device and mount it, or not.
As with other removable disk technology, it is important that the operating system is not trying to make use of the disk before removing it. On Linux instances this typically involves unmounting any file systems mounted from the volume. The OpenStack volume service cannot tell whether it is safe to remove volumes from an instance, so it does what it is told. If a user tells the volume service to detach a volume from an instance while it is being written to, you can expect some level of file system corruption as well as faults from whatever process within the instance was using the device.
There is nothing OpenStack-specific in being aware of the steps needed to access block devices from within the instance operating system, potentially formatting them for first use and being cautious when removing them. What is specific is how to create new volumes and attach and detach them from instances. These operations can all be done from the Volumes page of the dashboard or by using the cinder command-line client.
To add new volumes, you need only a name and a volume size in gigabytes. Either put these into the create volume web form or use the command line:
$ cinder create --display-name test-volume 10
This creates a 10 GB volume named "test-volume." To list existing volumes and the instances they are connected to if any:
$ cinder list +------------+---------+--------------------+------+-------------+-------------+ | ID | Status | Display Name | Size | Volume Type | Attached to | +------------+---------+--------------------+------+-------------+-------------+ | 0821...19f | active | test-volume | 10 | None | | +------------+---------+--------------------+------+-------------+-------------+
OpenStack Block Storage also allows for creating snapshots of volumes. Remember that this is a block level snapshot that is crash consistent, so it is best if the volume is not connected to an instance when the snapshot is taken and second best if the volume is not in use on the instance it is attached to. If the volume is under heavy use, the snapshot may have an inconsistent file system. In fact, by default, the volume service does not take a snapshot of a volume that is attached to an image, though it can be forced to. To take a volume snapshot either select Create Snapshot from the actions column next to the volume name on the dashboard volume page, or run this from the command line:
usage: cinder snapshot-create [--force <True|False>] [--display-name <display-name>] [--display-description <display-description>] <volume-id> Add a new snapshot. Positional arguments: <volume-id> ID of the volume to snapshot Optional arguments: --force <True|False> Optional flag to indicate whether to snapshot a volume even if its attached to an instance. (Default=False) --display-name <display-name> Optional snapshot name. (Default=None) --display-description <display-description> Optional snapshot description. (Default=None)
If a user tries to create a volume and the volume immediately goes into an error state, the best way to troubleshoot is to grep the cinder log files for the volume's UUID. First try the log files on the cloud controller and then try the storage node where the volume was attempted to be created:
# grep 903b85d0-bacc-4855-a261-10843fc2d65b /var/log/cinder/*.log
