You create a snapshot virtual disk to provide host access to a snapshot image within a snapshot group. A read-write snapshot virtual disk has its own repository that is used to save any subsequent modifications made by the host application to the base virtual disk without affecting the referenced snapshot image. The snapshot virtual disk can be designated as either read-only or read-write:
• A read-only snapshot virtual disk provides a host application with READ access to a copy of the data contained in the snapshot image, but without the ability to modify the snapshot image. A read-only snapshot virtual disk does not have an associated repository.
• A read-write snapshot virtual disk requires an associated repository to provide the host application with WRITE access to a copy of the data contained in the snapshot image.
Snapshot Virtual Disk Limitations
• You cannot create a snapshot virtual disk of a Failed base virtual disk.
• Snapshot repositories are fully resizeable. If you have the storage capacity you can increase the size of the snapshot repository to avoid a repository full message. Conversely, if you find that the snapshot repository is larger than you need, you can reduce its size to free up space that is needed by other logical virtual disks.
• If you create a snapshot virtual disk for a snapshot image and that snapshot image creation operation remains in a Pending state it is due to the following conditions:
– The base virtual disk that contains this snapshot image is a member of an asynchronous remote replication group.
– The base virtual disk is currently in a synchronizing operation. The snapshot image creation will complete as soon as the synchronization operation is complete.
Creating A Snapshot Virtual Disk
1. From the AMW, select the Storage & Copy Services tab.
2. Do one of the following:
• Select a base virtual disk, and then select Copy Services → Snapshot Virtual disk → Create. The
Select Existing Snapshot Image or New Snapshot Image window is displayed.
• Select a base virtual disk, and then select Copy Services → Snapshot Image → Create Snapshot
Virtual Disk. The Snapshot Virtual Disk Settings window is displayed. Go to step 4.
3. If you selected a base virtual disk in step 1, choose the snapshot image for which you want to create
a snapshot virtual disk. Do one of the following:
• Select A new snapshot image (on an existing snapshot group) and then a snapshot group from the existing snapshot group table and then click Next.
The Snapshot Virtual Disk Settings window is displayed.
4. In the Snapshot virtual disk name field, enter a unique name (30 character maximum) that best
describes the virtual disk selected for this snapshot image, for example, AccountingData. By default, the snapshot virtual disk name is shown in the name text box as follows:
[base-virtual disk-name] - SV + sequence-number
In this example, SV (snapshot virtual disk) is the appended suffix and sequence-number is the chronological number of the snapshot virtual disk relative to the base virtual disk.
For example, if you create the first snapshot virtual disk for a base virtual disk called “Accounting”, the default name of the snapshot virtual disk is “Accounting_SV_01”. The default name of the next snapshot virtual disk you create based on “Accounting” is “Accounting_SV_02”.
There is a 30-character limit. After you reach this limit, you can no longer type in the text box. If the base virtual disk is 30 characters, the default name for the group uses the base virtual disk name truncated enough to add the suffix “SV” and the sequence string.
5. In the Map to host drop-down, specify how you want to map the host to the snapshot virtual disk.
• Map Now to Default Group – The virtual disk is automatically assigned a logical unit number (LUN) and is accessible by any hosts that are connected to the storage array.
• Map Later – The virtual disk is not assigned a LUN and is not accessible by any hosts until you go to the Host Mappings tab and assign a specific host and LUN to this virtual disk.
• Select a specific host – You can select a specific host or host group from the list. This option is available only if Storage Partitioning is enabled.
NOTE: Make sure there are enough free LUNs on the host or host group that you selected to map to a snapshot virtual disk.
6. Select how to grant host access to the snapshot virtual disk. Do one of the following:
• Select Read Write and go to step 7.
• Select Read Only and click Finish to create the snapshot virtual disk. Go to step 8.
NOTE: Repositories are not required for Read Only snapshot virtual disks.
Keep these guidelines in mind when you grant host access to a snapshot virtual disk:
• Each host has its own logical unit number (LUN) address space and allows the same LUN to be used by different host groups or hosts to access snapshot virtual disks in a storage array. • You can define one mapping for each snapshot virtual disk in the storage array.
• Mappings are shared between controllers in the storage array.
• The same LUN cannot be used twice by a host group or a host to access a snapshot virtual disk. You must use a unique LUN.
• An access virtual disk mapping is not required for out-of-band storage arrays.
7. Choose how you want to create the repository for the Read-Write snapshot virtual disk. Do one of
the following:
• Select Automatic and click Finish to create the snapshot virtual disk repository with the default capacity settings. This option is the recommended one.
• Select Manual and click Next to define the properties for the snapshot virtual disk repository. Then click Finish to continue with the snapshot virtual disk creation procedure.
Use this option if you want to specify all of the customizable settings for the snapshot virtual disk repository. The Manual method is considered advanced and only those who understand physical disk consistency and optimal physical disk configurations should use this method.
8. Click Finish.
The snapshot virtual disk and its properties under the individual virtual disk node for the associated base virtual disk is displayed in the navigation tree. The snapshot virtual disk is added as a new virtual disk that contains the snapshot image information, which is the data of the virtual disk at the particular time of snapshot image creation.