The VSS is a key feature of the DPM backup and restore processes for your Microsoft production environment. For a few minutes you will get a deep dive into how VSS works and "what makes it tick".
VSS was first introduced in the Windows Server 2003 release and has been developed since. The VSS enables you to make a backup of your production servers while they are still running their production processes.
The VSS consists of four different blocks:
The VSS requester: The DPM agent is a requester and the purpose of this is to initiate a request for a snapshot to happen.
The VSS writer: SQL, Exchange, SharePoint, and so on all have a defined VSS writer. The VSS writer guarantees that there is a consistent data set for backup.
The VSS provider: The VSS provider is software- or hardware-based. The VSS provider creates and maintains the shadow copies. By default, you are using a software provider that resides within the operating system. The software provider uses a copy-on-write technique that will be explained shortly.
The VSS service: To make the requester, writer, and provider work together, you will need a coordination service. The VSS service is the coordinator that makes the communication between the different components work.
Let's have a look at how the different components of the shadow copy services interact with each other to be able to make a consistent shadow copy of your production environment. The following diagram is a graphical explanation of the process:
The DPM agent sends a query to the VSS to enumerate the writers and the writer metadata within the protected servers' operating system and prepare for the creation of a shadow copy:
The VSS writer creates an XML file that will describe the components and data stores that need to be included in the backup and also a definition of the restore process. The information is transferred to the VSS that will provide the VSS Requestor with the VSS writer's description. The VSS Requestor will select the components for the backup process.
The VSS will receive the VSS Requestor's choice for backup and will instruct the VSS writers to prepare their data for creating a shadow copy.
The VSS writer will complete all open transactions, rolling transaction logs, and flushing caches. When this process is done, the VSS writer notifies the VSS that the data is ready to be shadow copied.
The VSS instructs the VSS writers to freeze their write I/O requests for that specific application. During the freeze state, the shadow copy is created. This takes just a few seconds but there is a time-out limit of 60 seconds. The shadow copy service will flush the file system buffer and freeze the filesystem. This process makes the recording of the system metadata and verifies that it is correct and that the data that will be shadow copied is written in a consistent order.
The VSS initiates the provider to create a shadow copy. This takes 10 seconds and, during this time, the write I/O is frozen. However, you are still able to read the data being processed.
The VSS releases the file system write I/O.
The VSS tells the application to un-freeze the I/O requests.
If any error occurs then the requester can retry the process.
If the shadow copy creation was successful the VSS returns the location of the files to the VSS Requestor.
When the VSS coordinates a creation of a shadow copy, there are three different techniques to achieve this:
Complete copy: This technique makes a full copy or a clone of a disk
Copy-on-write: This is a technique that only copies data that has changed and is used by the DPM
Redirect-on-write: When the original volume receives a change, the change is made to another volume that stores the shadow copy storage area