Concepts

When a virtual machine looks at its disk, it sees something which looks like a real physical disk. This is an illusion. In reality the bytes of data written to the "virtual disk" probably reside in a file in a filesystem or in a logical volume on some physical storage system that the VM cannot see.

We call the real physical storage system a Storage Repository (SR).

We call the virtual disks within the SR volumes.

Manipulating volumes

When a VM is installed, a volume will be created. Typically this volume will be deleted when the VM is uninstalled. The Xapi toolstack doesn't know how to manipulate volumes on your storage system directly; instead it delegates to "Volume plugins": implementation-specific plugins which know how to talk the storage-specific APIs. These volume plugins can be anything from simple scripts in domain 0 to sophisticated services running somewhere on the network.

Consider for example a system using Linux LVM, where individual LVs are mapped to VMs as virtual disks. The volume plugin for LVM could implement the Volume.create API by simply calling

  lvcreate -n name -L 64GiB -Z n
 

Consider another example where volumes are simple sparse files stored on an NFS share. The volume plugin could implement the Volume.create API by simply calling:

  dd if=/dev/zero of=disk.name count=1 skip=64G

Connecting volumes to VMs

VMs running on the Xen hypervisor use special shared-memory protocols to access their disks and network interfaces. There are several implementations of these protocols including:

• the Linux xen-blkback kernel module
• the FreeBSD blkback driver
• the QEMU qdisk userspace implementation
• the XenServer tapdisk3 userspace implementation
• the MirageOS userspace and kernelspace implementation

With so many implementations to choose from, which one should we use for a given volume? This decision - and how to configure the implementation for maximum performance - is the job of the Datapath plugin.

Disks as URIs

Every volume has one or more URIs, which describe how to access the data within the volume. Examples include:

• file:///local/block/device - a local file or block device
• nfs://server/export/path/file.qcow2 - a remote NFS server exporting a .qcow2 format file
• smb://server/export/path/file.vhd - a remote CIFS server exporting a .vhd format file
• rbd://pool/volume - a volume in a Ceph storage pool
• iscsi://target/lun - a LUN on an iSCSI array

The Xapi toolstack takes the list of URIs provided by the Volume plugin and creates a connection between the VM and the disk. Xapi chooses a "Datapath plugin" based on the URI scheme. The Datapath plugin returns Xen-specific connection details, including choice of backend (kernel blkback or userspace qemu) and caching options.