VBS Architecture

The job of VBS is done on two ends: the volume server, the Xen manager, and through the interactions between the two. So our architecture is composed of three components: the VBS web service, the volume server delegate web service, and the VMM delegate web service.

The VBS web service is the front service that answers the requests from VBS clients, and coordinates the operations on the volume server and Xen VMMs. It provides the following method interfaces to the VBS client:
- create-volume
- delete-volume
- describe-volumes
- describe-volume
- create-snapshot
- delete-snapshot
- describe-snapshots
- describe-snapshot
- attach-volume
- detach-volume

The volume sever delegate web service is the service sitting on the logical volume server and encapsulating the LVM and ISCSI command lines, which are necessary to complete VBS operations. It is called by the VBS web service, and completes the operations of creating/deleting volumes/snapshots, exporting/deleting ISCSI targets by executing LVM and ISCSI commands.

The VMM delegate web service is the service sitting on VMM nodes and encapsulating VMM and ISCSI command lines. It is called by the VBS web service, and completes the operations of discovering and importing ISCSI targets, and attaching and detaching the ISCSI device to/from Xen DomU VMs.

The work flow of VBS is controlled by the VBS web service to complete the functions. A typical work flow for the attach-volume operation is composed of the following invocations carried out by the VBS web service:
(1) call the volume server delegate service to export an ISCSI target for ;
(2) call the VMM delegate service on to discover the new ISCSI target, and log in to it. This will result in a new ISCSI device created on the VMM node;
(3) call the VMM delegate service to attach the corresponding ISCSI device to , which will be visible as inside the VM;
(4) update the metadata about the , so that consequent calls to describe-volume(s) about this volume will return consistent information.