About Virtual Disks and Snapshots
Updated: September 29, 2010
Applies To: Microsoft iSCSI Software Target
Microsoft iSCSI Software Target creates storage devices as iSCSI virtual disks. These iSCSI virtual disks offer flexible and effective storage. Virtual disks can be extended to provide extra capacity on demand, enable efficient storage utilization, minimize the time that is required to create disks, and minimize the down time that is typically required to install new disks.
iSCSI virtual disks are files in the virtual hard disk (VHD) format. iSCSI virtual disks are assigned to specific iSCSI targets, and only the iSCSI virtual disks assigned to an iSCSI target are available to an iSCSI initiator. After the iSCSI initiator logs on to the iSCSI target, these virtual disks appear as locally-attached hard disks. These disks can be locally mounted to support backup and recovery operations.
With Microsoft iSCSI Software Target, you can provide effective and efficient backup and recovery for virtual disks on the iSCSI target without stopping applications or closing files. You can then transport data to a central server for LAN-free and server-free backup. This eliminates the backup window that is needed for traditional backup solutions, and it allows application servers to continue running without disruption to clients. The primary functionality that is provided in iSCSI Software Target to support data backup and recovery includes the following:
A snapshot is a point-in-time, read-only copy of an iSCSI virtual disk that can be used for fast system recovery in the event of data corruption from a virus attack, system security compromise, or an inadvertent user deletion. These are especially useful for interim backups of data that has changed since the previous regular backup because they are differential copies.
Local disk access
Local access to data on virtual disks can be set up by mounting a read-only snapshot of an iSCSI virtual disk or mounting a virtual disk as a read-write disk on the local computer. This makes it possible to use other backup software to perform regular backups.
The rollback functionality in iSCSI Software Target makes it possible to roll back a virtual disk to a snapshot.
Using backup software requires appropriate coordination of actions between the backup software and iSCSI Software Target; for example, setting up appropriate schedules and configurations for creating and mounting snapshots, and including the locally mounted snapshots in routine backups.
Snapshots are useful for backup and recovery operations and for data mining. You can create a snapshot manually or set up automatic snapshots to be taken at specific times. Snapshots offer the following advantages:
Can be scheduled to be created automatically.
Offer space-efficiency because they are differential copies.
Provide fast system recovery of files and volumes if data is accidentally deleted by a user, overwritten, or corrupted by a malicious program.
Can be mounted locally or exported to facilitate backup and recovery operations.
Do not require that you close files or stop programs when you create them, so application servers can continue servicing clients without disruption.
iSCSI snapshots are created by using the Volume Shadow Copy Service and a storage array with a hardware provider that is designed for use with the Volume Shadow Copy Service. Snapshots that are created on the iSCSI target server are “crash consistent.” This means that if the iSCSI initiator computer stops responding, the state of the snapshot will be the same as the state of the virtual disk. Most modern file systems can recover from this state. To enable crash consistent snapshots, the Microsoft iSCSI Software Target VSS Hardware Provider, which is available as an option in iSCSI Software Target, must be installed on the iSCSI initiator computer. The hardware provider coordinates with the local Volume Shadow Copy Service to create a consistent image of the volume that can be transported to a central backup server.
With Microsoft iSCSI Software Target, you can make a snapshot available on remote computers by using the Export Snapshot Wizard. You can use the wizard to export the snapshot to one or more iSCSI targets. After you export a snapshot, it appears on the iSCSI initiator as a local disk with read-only access, and it can be mounted by using a drive letter. This is useful for making files available to users for a short period of time, such as might be required for search and recovery of a specific file.
With Microsoft iSCSI Software Target, you can roll back an iSCSI virtual disk to a previous snapshot. A rollback of the disk requires the use of a temporary directory (such as C:\WINDOWS\Temp). This directory must contain sufficient space to store the differential data. If there is insufficient space, the rollback will stop, and the virtual disk content will not change.
Using hardware providers
To manage iSCSI virtual disks and snapshots, you use the appropriate iSCSI Software Target Hardware Providers. These hardware providers, which are provided in the iSCSI Software Target package, include:
Microsoft iSCSI Software Target Virtual Disk Service Hardware Provider
Virtual Disk Service (VDS) is a set of application programming interfaces (APIs) that provides a single interface for managing disks. VDS provides an end-to-end solution for managing storage hardware and disks, and for creating volumes on those disks. The Microsoft iSCSI Software Target VDS Hardware Provider is required to manage virtual disks on a storage subsystem.
You install the Microsoft iSCSI Software Target VDS Hardware Provider on each iSCSI initiator computer that is running a storage management application (such as Storage Manager for SANs) that uses the hardware provider to manage storage, as described later in this guide.
Microsoft iSCSI Software Target Volume Shadow Copy Service Hardware Provider
iSCSI snapshots are created by using Volume Shadow Copy Service (VSS) and a storage array with a hardware provider that is designed for use with VSS. A Microsoft iSCSI Software Target VSS Hardware Provider is required to create transportable snapshots of iSCSI virtual disks and to create application consistent snapshots from iSCSI initiators.
You install this hardware provider on the iSCSI initiator server that you use to perform backups.
You can perform many of the administrative tasks for the Microsoft iSCSI Software Target from the command prompt by using standard Windows Server 2008 R2 utilities. Following are some commands that you might find useful.
Command line tools for VDS Hardware Provider
The following commands can be used to work with disks and volumes from the command prompt:
MountVol.exe: MountVol is used to create, delete, or display volume mount points.
DiskRaid.exe: DiskRaid is used to create, delete, and optimize physical and logical redundant arrays of independent disks (RAIDs).
Command line tools for VSS Hardware Provider
The following tools can be used to work with VSS from the command prompt:
VSSAdmin.exe: VSSAdmin is used to manage the Volume Shadow Copy Service, including creating, resizing, and deleting VSS allocations on volumes.
DiskShadow.exe: DiskShadow is a tool for working with VSS, similar to VSSAdmin, but it also lets you work with snapshots.
For more information on using command line tools, see Command-line reference, A-Z List (http://technet.microsoft.com/en-us/library/cc772390(WS.10).aspx) on Microsoft TechNet.