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Using the Volume Snapshot Feature

The snapshot feature can be used to take a snapshot of a mirrored or an unmirrored volume, but snapshot is not supported for RAID-5 volumes.


Note    To take a snapshot of a volume, you must have an empty dynamic disk in the volume’s disk group.

The snapshot feature enables quick and efficient online backup of volumes, with minimum disruption to the user. Snapshot automatically enables FastResync (FR) and creates a snapshot image of a volume, which can be detached from the volume. The detached snapshot image can be used for backup, or other purposes, while the original volume remains in service. The snapshot can then be quickly reattached and resynchronized with the volume, through the FR reconnection capability. The snapshot volume can also be permanently disconnected from the original volume.

You can attach more than one snapshot plex to a volume, by issuing successive snapstart commands on the volume. This enables you to take multiple snapshots of a single volume.

You can attach a snapshot to either the original volume or the snapshot volume.

The following five snapshot commands are supported by VxVM, and are accessed through the Volume context menu:

The five snapshot commands are subcommands of the Snap command on the Volume context menu.

Using Snapshot

In summary, the snapshot procedure consists of three steps:

  1. Attaching a snapshot mirror to a volume, using Snap Start.
  2. Detaching the snapshot mirror and creating the snapshot volume that is associated with the snapshot mirror. The Snap Shot command is used for this step.
  3. Either reattaching the snapshot to the volume after you are done using it, or permanently disconnecting it from the volume. Snap Back reattaches the snapshot mirror; Snap Clear permanently disconnects the snapshot mirror from the volume, removing the association between the snapshot volume and the original volume.

Snap Abort aborts the snapshot operation after issuing the Snap Start command.

Creating a Snapshot Copy of a Volume

Snap Start creates a snap mirror or plex, which is attached to and synchronized with a volume. Once the snap mirror is synchronized, its status, as displayed on the Mirrors tab of the GUI, changes to Snap Ready.

After the snap mirror is synchronized with the volume, it can be used as a snapshot mirror. The synchronization process can take a while, but it does not interfere with use of the volume. If the Snap Start process fails, the snapshot mirror is removed and its space is released. After the mirror is synchronized, it continues to be updated until it is detached.


Note    To create a snapshot mirror on a dynamic volume, you must have an empty dynamic disk in the dynamic disk group of that volume.

 
   To create a snapshot mirror, do the following:

  1. Right-click on the volume that you want to take a snapshot of.
  2. A context menu is displayed.
  3. Select Snap > Snap Start.
  4. A new snapshot mirror is attached to the volume.
    The new mirror is added to the Mirrors tab for the volume. The mirror is identified as a Snapshot plex and is identified with the Snapshot icon. After the mirror is synchronized with the volume, its status becomes Snap Ready.
    The DCO (disk change object) volume is created to track the regions on a volume that are changed while a mirror is detached. The DCO volume is not included in the tree view of the Volume Manager GUI. To view the DCO volume, you must use the Disk View. To access the Disk View, click the Disk View tab in the right pane or select Disk View from a disk’s context menu.

    Note    The Break Mirror and Remove Mirror commands do not work with the snapshot mirror.

Creating the New Volume

Snap Shot detaches the snapshot mirror and creates a new volume that is associated with that mirror. This process takes a relatively short time, typically less than a minute, during which users can be requested to not access the original volume. After the snapshot volume is detached, it can be used for backup or other purposes.

To take a snapshot, do the following:

  1. Right-click on the volume with the snapshot mirror.
  2. A context menu is displayed.
  3. Select Snap > Snap Shot.
  4. The Snapshot Volume dialog box is displayed. You are asked to select the snapshot mirror to be used to create the snapshot volume.
  5. Select the mirror and click OK.
  6. When there is only one snapshot mirror, “Automatic select mirror to add snapshot” will be highlighted. If there are multiple mirrors and you want to select the mirror to be used, click to select “Manual select which mirror to add snapshot.” You then can select the mirror from the list on the screen.
    Click OK.
    The snapshot mirror is detached from the original volume and a new volume is created that is associated with the snapshot mirror.
    The snapshot mirror is no longer displayed on the Mirrors tab for the original volume. The new snapshot volume is displayed under the Volumes folder in the tree view. The program assigns it the next available drive letter. You can now use the snapshot volume for backup or other purposes.

    Notes:




Merging a Snapshot Volume (Volume Snapback)

The volume Snap Back procedure merges a snapshot copy of a volume with the original volume. The snapshot plex is detached from the snapshot volume and attached to the original volume. The snapshot volume is removed. This task resynchronizes the data in the volume so that the plexes are consistent. By default, the data in the original plex is used for the merged volume.

To reattach a snapshot mirror to the original volume, do the following:

  1. Right-click the volume whose snapshot you want to reattach.
  2. A context menu is displayed.
  3. Select Snap > Snap Back.
  4. Specify whether the snapshot volume is resynchronized with the original volume, or the original volume is synchronized to the snapshot volume and click OK.

  5. Note    One situation where you might want to resynchronize to the snapshot volume rather than the original volume is the case where something has happened to the original volume so that its data integrity is no longer sound.
    The snapshot mirror is reattached to the original volume and the snapshot volume is deleted if it is the last plex of the volume. (If the snapshot volume has other mirrors associated with it, it is not deleted.)

Dissociating a Snapshot Volume (Volume Snap Clear)

The volume Snap Clear procedure permanently breaks the association between a snapshot and its original volume. The snapshot volume becomes an independent volume.

Snap Clear permanently removes a snapshot mirror from a volume. Snap Clear can be specified only after the snapshot command has been issued.

 
   To permanently remove a snapshot mirror from a volume, do the following:

  1. Right-click on the volume, whose snapshot mirror you are removing.
  2. A context mirror is displayed.
  3. Select Snap>Snap Clear.
  4. A message box is displayed asking you to confirm the snap clear operation for the specified volume.
  5. Click Yes.
  6. The snapshot mirror is permanently disconnected from the original volume and is no longer displayed on the Volume Manager console.

Aborting a Snapshot

The volume Snap Abort procedure allows you to abort the snapshot procedure before it completes. If you have started the procedure, through Snap Start, but have not yet clicked on Snap Shot to complete the procedure, you can select Snap Abort to abort the procedure.

 
   To abort a snapshot operation begun with snap start, do the following:

  1. Right-click on the volume that you want to abort the snapshot operation on.
  2. A context menu is displayed.
  3. Select Snap > Snap Abort.
  4. A message box is displayed asking you to confirm the snap abort operation for the specified volume.
  5. Click OK.
  6. The snapshot process is aborted.

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