How to Use the Command 'virsh domblklist' (with examples)
The virsh domblklist command is a tool from the libvirt suite that allows users to list information about block devices associated with a specific virtual machine. This is particularly useful for managing and troubleshooting virtual machines, as it provides insights into the storage configurations linked with them. The command is essential for system administrators and users who work with virtualization environments by offering a clear view of connected block devices, including their target names, source paths, disk types, and device values.
Use case 1: List the Target Name and Source Path of the Block Devices
Code:
virsh domblklist --domain vm_name
Motivation:
Understanding the target name and source path of block devices associated with a virtual machine is crucial for tasks such as storage management, backup operations, and performance optimization. By knowing the target names, administrators can efficiently map virtual devices to their corresponding physical or logical storage units. This command is often used when you need to quickly verify the disc association or when preparing for operations like data migration or snapshots that require accurate device mapping.
Explanation:
virsh: This is the base command to interact with the virtualization libraries provided by libvirt.domblklist: This subcommand specifically targets listing block devices related to a domain, which is another term for a virtual machine.--domain: This option specifies that the action should be taken on the domain (virtual machine) named immediately after it.vm_name: Replace this with the actual name or ID of the virtual machine for which you want to retrieve block device information.
Example Output:
Target Source
------------------------------------------------
vda /var/lib/libvirt/images/vm_name.img
vdb /var/lib/libvirt/images/extra_disk.img
This output shows two columns: the ‘Target’ column gives the virtual device name within the guest environment, and the ‘Source’ shows the actual file or device on the host machine that maps to the virtual device.
Use case 2: List the Disk Type and Device Value as well as the Target Name and Source Path
Code:
virsh domblklist --domain vm_name --details
Motivation:
This usage of the command is invaluable when an in-depth understanding of the storage configurations is needed, especially in complex virtual environments. By obtaining detailed information that includes the disk type and device value along with other data, administrators can perform advanced diagnostics, improve storage allocation strategies, and ensure compliance with policies designed to optimize storage utilization. Knowing disk types, such as ‘file’, ‘block’, etc., helps in identifying how data is stored while understanding device values aids in integration with other system tools.
Explanation:
virsh: Again, the base command for interacting with libvirt.domblklist: The subcommand to list block devices specific to a domain.--domain: Indicates the operation on a specified domain.vm_name: The specific virtual machine’s name or ID.--details: An additional flag that expands the output to include more detailed information about each block device. With this, apart from the target name and source path, you also get to see the disk type (e.g., ‘file’, ‘block’) and the specific device value (e.g., ‘disk’).
Example Output:
Type Device Target Source
-------------------------------------------
file disk vda /var/lib/libvirt/images/vm_name.img
block disk vdb /dev/sdb
In this output, the ‘Type’ column indicates the nature of the storage entity, ‘Device’ specifies whether it’s a disk or a CDROM, and ‘Target’ and ‘Source’ remain similar to the basic command output.
Conclusion
The virsh domblklist command is a potent tool for managing and understanding the storage layout of virtual machines. Whether you require a quick overview or a deep dive into the details, this command serves both requirements efficiently. The above use cases demonstrate how you can employ this command for various administrative tasks, providing clarity in virtualized storage environments. By mastering this command, administrators can significantly enhance their operational capabilities within any libvirt-managed infrastructure.
