Windows Backup Implementations

The following sections discuss issues when backing up Windows virtual machines.

Disabling Automount in Windows Proxy

When using HotAdd transport from a Windows proxy, it can make unwanted changes to HotAdded volumes. To prevent this, backup vendors and customers should disable Windows automount on the backup proxy.

To disable Windows automount

1	Start DiskPart.

C:\> diskpart

2	Disable automounting.

DISKPART> automount disable

3	Prevent any previously mounted volumes from being re-mounted the next time.

DISKPART> automount scrub

4	Exit DiskPart.

DISKPART> exit

Security and Remote Desktop

For security reasons, remote desktop protocol (RDP) should be disabled on a Windows proxy.

Working with Microsoft Shadow Copy

Microsoft Shadow Copy, also called Volume Snapshot Service (VSS), is a Windows Server data backup feature for creating consistent point-in-time copies of data (called shadow copies).

The type of quiescing used varies depending on the operating system of the backed-up virtual machine, as shown in Driver Type and Quiescing Mechanisms Used According to Guest Operating Systems. ESXi 4.1 added support for Windows 2008 guests using application level quiescing.

Driver Type and Quiescing Mechanisms Used According to Guest Operating Systems

Guest Operating System	Driver Type Used	Quiescing Type Used
Windows XP 32-bit Windows 2000 32-bit	Sync Driver	File-system consistent quiescing
Windows Vista 32- or 64-bit Windows 7 32- or 64-bit	VMware VSS component	File-system consistent quiescing
Windows 2003 32- or 64-bit	VMware VSS component	Application-consistent quiescing
Windows 2008 32- or 64-bit Windows Server 2008 R2	VMware VSS component	Application-consistent quiescing. For this to be available, several conditions must be met: ■ Virtual machine must be running on ESXi 4.1 or later. ■ The UUID attribute must be enabled. It is enabled by default for virtual machines created on 4.1 or later. For details see Enable Windows 2008 Virtual Machine Application Consistent Quiescing. ■ The virtual machine must use SCSI disks only and have as many free SCSI slots as the number of disks. Application-consistent quiescing is not supported for virtual machines with IDE disks. ■ The virtual machine must not use dynamic disks.
Windows Server 2012	VMware VSS component	Same as above.
Other guest operating system	Not applicable	Crash-consistent quiescing

File-system consistent quiescing prevents file systems from becoming corrupted, for example, journaled file systems are allowed to write out pending transactions. Crash-consistent quiescing is the ability to restore an application as if it suddenly crashed and lost all stateful information. This involves minimal effort because only data already written to disk is guaranteed safe. Application-consistent quiescing is the ability to restore stateful information as well.

Restore must be done using the backup application’s guest agent. The vSphere APIs for Data Protection provide no host agent support for this. Applications authenticating with SSPI might not work right because HTTP access will demand a user name and password, unless the session was recently authenticated.

When performing VSS quiescing while creating the snapshot of a Windows virtual machine, VMware Tools generate a vss-manifest.zip file containing the backup components document (BCD) and writer manifests. The host agent stores this manifest file in the snapshotDir of the virtual machine. Backup applications should get the vss-manifest.zip file so they can save it to backup media. There are several ways to get this file:

■	Using the datastore access HTTPS protocol, for example by browsing to https://<server-or-host>/folder/ and continuing downward to the snapshot directory until you find the vss-manifest.zip file.

■	By calling the CopyDatastoreFile_Task method in the vSphere API. This method accepts the URL formulated above for HTTPS, or a datastore path. (CopyVirtualDisk_Task is for VMDK files).

■	With the vifs command in the vMA or vCLI.

■	With the Copy-DatastoreItem cmdlet in the PowerCLI (requires PowerShell and VMware snap-in).

Windows 2008 application level quiescing is performed using a hardware snapshot provider. After quiescing the virtual machine, the hardware snapshot provider creates two redo logs per disk: one for the live virtual machine writes and another for the VSS and writers in the guest to modify the disks after the snapshot operation as part of the quiescing operations.

The snapshot configuration information reports this second redo log as part of the snapshot. This redo log represented the quiesced state of all the applications in the guest. This redo log must be opened for backup with VDDK 1.2 or later. The older VDDK 1.1 software cannot open the second redo log for backup.

Application consistent quiescing of Windows 2008 virtual machines is only available when those virtual machines are created in vSphere 4.1 or later. Virtual machines created in vSphere 4.0 can be updated to enable application consistent quiescing by modifying a virtual machine’s enableUUID attribute.

For information about VSS, see the Microsoft TechNet article, How Volume Shadow Copy Service Works. For information about Security Support Provider Interface (SSPI), see the MSDN Web site.

Enable Windows 2008 Virtual Machine Application Consistent Quiescing

1	Start the vSphere Client, and log in to a vCenter Server.

2	Select Virtual Machines and Templates and click the Virtual Machines tab.

3	Right-click the Windows 2008 virtual machine for which you are enabling the disk UUID attribute, and select Power > Power Off. Wait for the virtual machine to power off.

4	Right-click the virtual machine, and click Edit Settings.

5	Click the Options tab, and select the General entry in the settings column.

6	Click Configuration Parameters... The Configuration Parameters window appears.

7	Click Add Row.

8	In the Name column, enter disk.EnableUUID. In the Value column, enter TRUE.

9	Click OK and click Save.

10	Power on the virtual machine.

Application consistent quiescing is available for this virtual machine after the UUID property is enabled.

Application-Consistent Backup and Restore

Here is the approximate procedure for software to performs application-consistent backup and restore:

1	Call CreateSnapshot_task with the quiescent flag set true.

2	Open the leaf node of the disk with VDDK and read both the base VMDK and the snapshot at once.

3	Delete the snapshots created in the first step.

4	During restore, create a new virtual machine.

5	Write the VMDK to disk with VDDK. It should have both base and quiesced information.

During backup, if the snapshot was created with quiesce flag set to true, and all the quiescing conditions are met, so the snapshot is created involving VSS and the snapshot disks represent application consistent state of the guest OS. You should be able to confirm this by downloading the VSS manifest zip file, unzipping it to check if it has just the backup component document (in which case file system quiescing was performed) or also writer manifests (in which case application quiescing was performed).

Quiescing involves the VSS mechanism designed by Microsoft. So, regarding VSS backup-restore verification, refer to the VSS documentation provided by Microsoft. VMware helps by providing a vss-manifest.zip file that contains Backup/Writers Components details. This is generated by the VSS mechanism after backup. By cross verifying these backup/writers components details according to Microsoft VSS documentation, you can verify if a particular application-consistent quiescing was completed successfully or not.

VMware Tools is responsible for initiating the VSS snapshot process as the VSS requester. Users send a request to hostd for a quiesced snapshot of the virtual machine. The request goes from hostd to the VMware Tools for a VSS snapshot. Once the VSS snapshot is completed (with success or error) it communicates back to the hostd process. The VSS snapshot is created with the vss-manifest file, or without this file in the error case.

The VSS requester sets up the overall configuration for the backup operation, including whether the snapshot should be performed in component mode or not, whether to take a snapshot with a bootable system state, and whether the snapshot should be for a full copy or differential backup. If application-consistent quiescing is performed, then all writers and all components are involved.

VMware Tools initiates VSS quiescing using VSS_CTX_BACKUP context for application quiescing capable guests with backup state set to select components, backup bootable system state with backup type VSS_BT_COPY and no partial file support and VSS_CTX_FILE_SHARE_BACKUP for file system quiescing capable guests. Currently there is no way to control any of these parameters.

The VMware VSS Implementation

On Windows Server 2008, disk UUIDs must be enabled for VSS quiesced snapshots. Disk UUIDs might not be enabled if a virtual machine was upgraded from virtual hardware version 4.

VMware VSS does not support virtual machines with IDE disks, nor does it support virtual machines with an insufficient number of free SCSI slots.

Before vSphere 5.1, reverting to a writable snapshot sometimes left orphaned virtual disks that the system never removed. In the vSphere 5.1 release, writable snapshots are correctly accounted for as sibling snapshots. This permits cleaner management, because the disk chain matches the snapshot hierarchy, and it avoids orphaned disks. Linux backup software takes a read-only snapshot so is not affected. On Windows, VSS backup software may create two snapshots, one made writable by calling CreateSnapshot_task with the quiesce flag set true.

To add support for granular application control, specify:

■	whether pre-freeze and post-thaw scripts get invoked

■	whether quiescing gets invoked

■	VSS snapshot context (application, file system quiescing, and so forth)

■	VSS backup context (full, differential, incremental)

■	writers/components to be involved during quiescing

■	whether to fail quiescing or continue if one of the writers fails to quiesce

■	retry count

A VSS quiesced snapshot reports as VSS_BT_COPY to VSS, hence no log truncation. The VSS manifest can be downloaded with HTTP. By default, all VSS writers are involved, but a mechanism for excluding writers exists; see the VMware KB article 1031200. For help troubleshooting, see KB article 1007696.