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Creating and Scheduling Snapshots of Virtual Disks

Updated: September 29, 2010

Applies To: Microsoft iSCSI Software Target

iSCSI snapshots are created by using Volume Shadow Copy Service and a storage array with a hardware provider that is designed for use with 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 consistent snapshots, Microsoft iSCSI Software Target requires the Microsoft iSCSI Software Target VSS Hardware Provider, which is available as an option in iSCSI Software Target, to 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.

You can create a crash consistent snapshot in two ways:

  • Manually create a snapshot of a single virtual disk in the Microsoft iSCSI Software Target console. Manual creation of a snapshot is useful prior to performing a system upgrade, installation of new software, or any event that might corrupt the iSCSI virtual disk. If the upgrade or installation fails, the iSCSI virtual disk can be rolled back to the snapshot.

  • Use the Schedule Snapshot Wizard to set up a schedule for creating a single snapshot or recurring snapshots automatically. Regularly scheduled snapshots are useful for providing ongoing, interim backups between the backups that are performed using other backup software.

noteNote
Application consistent snapshots are created by using the VSS Hardware Provider and non-Microsoft VSS-aware backup software from the iSCSI initiator computer.

To schedule crash consistent snapshots, you need to specify the following:

  • Schedule Actions

    In addition to scheduling when to take a snapshot, you can specify that the snapshot should be mounted locally. For more information about locally mounted snapshots, see Mounting a Read-Only Snapshot Locally.

  • Resource Group

    In a failover cluster environment, the schedule is managed by the resource group. For more information about resource groups, see Configuring iSCSI Storage for High Availability.

  • Name

    A short name to identify the scope of the schedule. For instance, Weekly All Disks.

  • Virtual Disks

    The virtual disks for which snapshots are to be taken. You can include all virtual disks or select one or more individual virtual disks.

  • Frequency

    How often the snapshots should be taken (daily, weekly, monthly, or one time only).

  • Schedule

    The days and times when the snapshots should be taken (based on the frequency selected). Daily snapshots can be taken every day, weekdays only, or every specified number of days. Weekly snapshots can be taken for one or more weeks on any day of the week. Monthly snapshots can be taken every month or only for selected months, on any day of the month or on a specific milestone (such as the first Monday of the month).

    We recommend that snapshots be taken of all iSCSI virtual disks once every four hours; once before the start of the day, once in the middle of the day, and once at the end of the day.

noteNote
The Volume Shadow Copy Service (VSS) will automatically select a location to store the snapshots unless you define the location for each volume. To avoid potential issues, you should manually select the storage location to ensure that there is sufficient free disk space for the snapshots. In a failover cluster environment, snapshots should be stored in the same storage group as the virtual disk.

To set the storage location by using the Microsoft iSCSI Software Target console:

  1. Under Devices, right-click the virtual disk that you want to modify, and then select Properties.

  2. Click the Snapshot Storage tab.

  3. In Storage drive, select the drive where you want to store the snapshots.

  4. Under Maximum storage size, select No Limit or Use Limit, and then provide the number of megabytes to permit for the snapshot storage. Click OK.

After you create a snapshot schedule, you can modify these and other schedule settings. For more information, see Modifying a Snapshot Schedule.

To create an application-consistent snapshot (which means the data is in a state that an application can use it for recovery), the snapshot must be created from the iSCSI initiator computer by using the Microsoft iSCSI Software Target VSS Hardware Provider and backup software that works with Volume Shadow Copy Service.

By default, Volume Shadow Copy Service automatically selects a location for the snapshots. On the NTFS file system volume, this location is the System Volume Information folder. If you are using snapshots in a cluster, the location must be on the same shared drive as the virtual disk.

To modify the location and size of the storage to be used for snapshots, click Properties for the volume, click the Shadow Copies tab, and then click Settings.

Volume Shadow Copy Service requires that the NTFS volume have at least 300 MB of free space (the default) available on the system to store the snapshots. When the maximum size of the storage area is exceeded, the oldest snapshot is deleted.

CautionCaution
It is important to provide sufficient storage for snapshots. If a snapshot is deleted, it cannot be recovered.

Each volume can have up to 512 snapshots for iSCSI virtual disks. For example, if there are three iSCSI virtual disks residing on drive C:, the combined number of snapshots for all three iSCSI virtual disks is limited to 512. Snapshots are space-efficient because they are differential copies.

Membership in the local Administrators group, or equivalent, is the minimum required to complete this procedure. Review the details in "Additional considerations" in this topic.

To manually create a snapshot

  1. In Microsoft iSCSI Software Target, in the console tree, click Devices.

  2. In the results pane, right-click the virtual disk for which you want to create a snapshot, and then click Create Snapshot.

  3. After the snapshot is created, click OK.

To create a schedule for automatic snapshots

  1. In Microsoft iSCSI Software Target, in the console tree, double-click Snapshots, right-click Schedules, and then click Create Schedule.

  2. Follow the steps in the Schedule Snapshot Wizard.

noteNote
If you need read access to this snapshot on the local computer, be sure to use the Take a snapshot of the virtual disks and mount the snapshot locally option on the Schedule Actions page. If this snapshot is going to be backed up as part of a regular server backup procedure, ensure that the snapshot is scheduled to be created prior to the scheduled backup operation.

Additional considerations

  • You must be a member of the local Administrators group to perform these tasks.

  • To open iSCSI Software Target, click Start, point to Administrative Tools, and then click Microsoft iSCSI Software Target. (Another way to open iSCSI Software Target is to click Start, click Run, and then type iscsitarget.msc.)

Additional references

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