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The Volume Shadow Copy Service flushes the file system buffers and then freezes the file system, which ensures that the file system metadata is recorded correctly and the data to be shadow-copied is written in a consistent order.
The Volume Shadow Copy Service tells the provider to create the shadow copy. At this point applications are free to resume writing data to the disk that is being shadow-copied.
The shadow copy creation can be aborted if the writers are kept in the freeze state for longer than 60 seconds or if the providers take longer than 10 seconds to commit the shadow copy. The requester can retry the process go back to step 1 or notify the administrator to retry at a later time. If the shadow copy is successfully created, the Volume Shadow Copy Service returns the location information for the shadow copy to the requester.
In some cases, the shadow copy can be temporarily made available as a read-write volume so that VSS and one or more applications can alter the contents of the shadow copy before the shadow copy is finished. After VSS and the applications make their alterations, the shadow copy is made read-only. This phase is called Auto-recovery, and it is used to undo any file-system or application transactions on the shadow copy volume that were not completed before the shadow copy was created.
A hardware or software shadow copy provider uses one of the following methods for creating a shadow copy:. Complete copy This method makes a complete copy called a "full copy" or "clone" of the original volume at a given point in time. This copy is read-only. Copy-on-write This method does not copy the original volume. Redirect-on-write This method does not copy the original volume, and it does not make any changes to the original volume after a given point in time.
Instead, it makes a differential copy by redirecting all changes to a different volume. After the mirror connection is broken, the original volume and the shadow copy volume are independent.
The shadow copy storage area can be on the same volume or a different volume. This preserves a copy of the data block on the original volume before the change overwrites it. The copy-on-write method is a quick method for creating a shadow copy, because it copies only data that is changed. The copied blocks in the diff area can be combined with the changed data on the original volume to restore the volume to its state before any of the changes were made.
If there are many changes, the copy-on-write method can become expensive. Instead, the change is written to another volume's shadow copy storage area. Like the copy-on-write method, the redirect-on-write method is a quick method for creating a shadow copy, because it copies only changes to the data.
The copied blocks in the diff area can be combined with the unchanged data on the original volume to create a complete, up-to-date copy of the data. There are two types of shadow copy providers: hardware-based providers and software-based providers. There is also a system provider, which is a software provider that is built in to the Windows operating system.
Hardware-based shadow copy providers act as an interface between the Volume Shadow Copy Service and the hardware level by working in conjunction with a hardware storage adapter or controller. The work of creating and maintaining the shadow copy is performed by the storage array. Hardware providers always take the shadow copy of an entire LUN, but the Volume Shadow Copy Service only exposes the shadow copy of the volume or volumes that were requested.
A hardware-based shadow copy provider makes use of the Volume Shadow Copy Service functionality that defines the point in time, allows data synchronization, manages the shadow copy, and provides a common interface with backup applications. However, the Volume Shadow Copy Service does not specify the underlying mechanism by which the hardware-based provider produces and maintains shadow copies.
These providers are implemented as a user-mode DLL component and at least one kernel-mode device driver, typically a storage filter driver.
Unlike hardware-based providers, software-based providers create shadow copies at the software level, not the hardware level. A software-based shadow copy provider must maintain a "point-in-time" view of a volume by having access to a data set that can be used to re-create volume status before the shadow copy creation time.
An example is the copy-on-write technique of the system provider. However, the Volume Shadow Copy Service places no restrictions on what technique the software-based providers use to create and maintain shadow copies. A software provider is applicable to a wider range of storage platforms than a hardware-based provider, and it should work with basic disks or logical volumes equally well.
A logical volume is a volume that is created by combining free space from two or more disks. In contrast to hardware shadow copies, software providers consume operating system resources to maintain the shadow copy.
One shadow copy provider, the system provider, is supplied in the Windows operating system. Although a default provider is supplied in Windows, other vendors are free to supply implementations that are optimized for their storage hardware and software applications. To maintain the "point-in-time" view of a volume that is contained in a shadow copy, the system provider uses a copy-on-write technique.
Copies of the blocks on volume that have been modified since the beginning of the shadow copy creation are stored in a shadow copy storage area. The system provider can expose the production volume, which can be written to and read from normally. When the shadow copy is needed, it logically applies the differences to data on the production volume to expose the complete shadow copy. For the system provider, the shadow copy storage area must be on an NTFS volume.
The Windows operating system includes a set of VSS writers that are responsible for enumerating the data that is required by various Windows features.
In addition to backing up application data and system state information, shadow copies can be used for a number of purposes, including the following:. This is a fast-recovery scheme that allows an application administrator to restore data from a shadow copy to the original LUN or to a new LUN.
The shadow copy can be a full clone or a differential shadow copy. In either case, at the end of the resync operation, the destination LUN will have the same contents as the shadow copy LUN. During the resync operation, the array performs a block-level copy from the shadow copy to the destination LUN. While the resync operation is in progress, read requests are redirected to the shadow copy LUN, and write requests to the destination LUN.
This allows arrays to recover very large data sets and resume normal operations in several seconds. In a LUN swap, the shadow copy is imported and then converted into a read-write volume.
In LUN resynchronization, the shadow copy is not altered, so it can be used several times. In LUN swapping, the shadow copy can be used only once for a recovery. For the most safety-conscious administrators, this is important. When LUN resynchronization is used, the requester can retry the entire restore operation if something goes wrong the first time.
For this reason, the shadow copy LUN must use the same quality of storage as the original production LUN to ensure that performance is not impacted after the recovery operation. If LUN resynchronization is used instead, the hardware provider can maintain the shadow copy on storage that is less expensive than production-quality storage.
All of the operations listed are LUN-level operations. If you attempt to recover a specific volume by using LUN resynchronization, you are unwittingly going to revert all the other volumes that are sharing the LUN. Shadow Copies for Shared Folders uses the Volume Shadow Copy Service to provide point-in-time copies of files that are located on a shared network resource, such as a file server.
With Shadow Copies for Shared Folders, users can quickly recover deleted or changed files that are stored on the network. Because they can do so without administrator assistance, Shadow Copies for Shared Folders can increase productivity and reduce administrative costs.
With a hardware provider that is designed for use with the Volume Shadow Copy Service, you can create transportable shadow copies that can be imported onto servers within the same subsystem for example, a SAN.
These shadow copies can be used to seed a production or test installation with read-only data for data mining. With the Volume Shadow Copy Service and a storage array with a hardware provider that is designed for use with the Volume Shadow Copy Service, it is possible to create a shadow copy of the source data volume on one server, and then import the shadow copy onto another server or back to the same server. This process is accomplished in a few minutes, regardless of the size of the data.
The transport process is accomplished through a series of steps that use a shadow copy requester a storage-management application that supports transportable shadow copies. Import the shadow copy to a server that is connected to the SAN you can import to a different server or the same server. A transportable shadow copy that is created on Windows Server cannot be imported onto a server that is running Windows Server or Windows Server R2. A transportable shadow copy that was created on Windows Server or Windows Server R2 cannot be imported onto a server that is running Windows Server However, a shadow copy that is created on Windows Server can be imported onto a server that is running Windows Server R2 and vice versa.
Shadow copies are read-only. It works only if there is a hardware provider on the storage array. Shadow copy transport can be used for a number of purposes, including tape backups, data mining, and testing. In the case of a hard disk drive backup, the shadow copy created is also the backup.
Data can be copied off the shadow copy for a restore or the shadow copy can be used for a fast recovery scenario—for example, LUN resynchronization or LUN swapping.
When data is copied from the shadow copy to tape or other removable media, the content that is stored on the media constitutes the backup. The shadow copy itself can be deleted after the data is copied from it. It depends on the backup software that you used.
Most backup programs support this scenario for data but not for system state backups.
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