[Figure] Mirroring Configuration

Mirror Mode

Mirror mode employs both asynchronous and synchronous mirroring schemes. Understanding the advantages and disadvantages between synchronous and asynchronous mirroring is essential to the correct operation of this.

Async mode

With asynchronous mirroring, each write is captured and a copy of this is made. That copy is queued to be transmitted to the target system as soon as the network will allow it. Meanwhile, the original write request is committed to the underlying storage device and control is immediately returned to the application that initiated the write. At any given time, there may be write transactions waiting in the queue to be sent to the target machine.  But it is important to understand that these writes reach the target volume in time order, so the integrity of the data on the target volume is always a valid snapshot of the source volume at some point in time.  Should the source system fail, it is possible that the target system did not receive all of the writes that were queued up, but the data that has made it to the target volume is valid and usable.

Sync mode

With synchronous mirroring, each write is captured and transmitted to the target system to be written on the target volume at the same time that the write is committed to the underlying storage device on the source system.  Once both the local and target writes are complete, the write request is acknowledged as complete and control is returned to the application that initiated the write.  With synchronous mirroring, each write is intercepted and transmitted to the target system to be written on the target volume at the same time that the write is committed to the underlying storage device on the source system.  Once both the local and target writes are complete, the write request is acknowledged as complete and control is returned to the application that initiated the write.

Example of Shared Disk DR Configuration

Following steps are recommended to manage shared disk DR resource through Data replication software after installing MCCS.

Assume that a driver letter S: is a shared disk DR resource in a cluster. 

1. Install Data replication software if it has not been installed on node DR. 
2. While both nodes (assumed as node A and node B) are in the power off state, bring power online on node B first.
3. Install MCCS if it has not been installed and reboot the Node B after the installation.
4. Add driver S: to Data Replication Program though ‘%ExtMirrBase%\emcmd . setconfiguration S 256’ command.
5. Reboot node B. It will come up with the S: drives locked. 
6. Power on node A when it is checked driver S: is locked to node B
7. Install MCCS if it has not been installed and reboot the Node A after the installation.
8. Node A will come up with the S: drives writable. 
9. Use the MCCS console to create shared disk DR resource from node A S:(source) to node DR S:(target). 

Maintain the shared disk state to allow one disk access from one node and keep them locked to prevent the damage of data.

Adding

Add the DR shared disk resource. 

1. When add a resource from a group name, select a group → right click → 'Add Resource'. Or select Edit(E) from the main menu bar → select 'Add Resource'. Or select 'Add Resource' icon from the tool bar.   
   
2. Select 'SharedDisk DR' from Resource Type lists and click 'Next' button. When add a resource from the SharedDisk DR resource type name, this step will be skipped.
   
3. Enter the resource name and select driver letter of shared disk in the cluster.

4. Enter the IP address of data replication on the off-site DR node.

   The drive letter to be used as the shared disk DR should be same on the nodes. Data will be replicated from the source-primary node to the off-site DR node(Remote server). The remote server cannot be the source when the cluster is normal state. Be careful when you select the drive letter where data exists in the remote server, the data will be overwritten when mirror set is completed. If you try to add an existing mirror disk between shared disk set and remote disk, <mirrored> will be followed to the driver letter of shared disk.

   
   

   
     [Figure] Adding Shared Disk DR Resource

   
   

5. As 'Additional Settings', Mirror Type and Check Disk option is available and click 'OK' button.

   <MirrorType> 

   You cannot change the mode later for the volume. The only way to do this is that delete the mirror disk resource and add again.

    
   <ChkDsK > 

   Set the check disk command option. For more details on the function, refer to MSDN.

    
    [Figure] Defining mirror type and check disk option

   
   

6. When click OK button, following warning message will be displayed and click 'OK' button if the informations are correct.

   
   [Figure] Alert popup message after click OK button   
    

7. Click 'Finish' button to add the shared disk DR resource.

Deleting

Select the resource -> right click -> delete the resource.

State

The following table explains the state of shared disk DR resource.

Attribute

Show the specified attribute of the shared disk DR resource.

Specified attribute values are normally entered when add the resource, This value can be edited or viewed through 'Specified Attribute' table in the Resource Attributes tab on the detailed information panel.