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Mirroring is classified into a sync mode and an async mode depending on the time of response to a request for saving.

Async mode

A response to writing on the mirror volume is immediately sent after the source volume is saved. Since it does not wait until data is sent to the target volume and saved there, the response time is very fast. However, in case of trouble, it can cause data mismatches due to data failed to be transmitted at that time.

Sync mode

When there is a request for writing to the disk, it will be written to the target volume as soon as it is written to the source volume. The final, successful response to a request for writing is achieved after both the source and the target volume are written. The response time is slow, but there exists the same data for the source and the target all the timesWith 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.

Add

Add the DR shared disk resource. Before you can add the shared disk DR resource, you must install a copy tool at the remote server that data will be copied to.

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1. Select the group -> Right click with the mouse -> Add the resource.   
   
2. In resource wizard, select a shared disk DR and click the Next button. 
   

3. The shared volume set of resource wizard will display the drive characters that can be used to compose the shared disk DR and the existing drive characters that are already configured.
   You cannot inquire about information at the remote server. So, you need to enter IP addresses on your own. 
   

   Warning
   The drive characters to be used as the shared disk DR should be same on the both servers that compose a cluster. Data will be copied to the target node from the source node. The remote server cannot be the source. Be careful when you select the drive character where data exists. If you try to add the existing mirror volume, <mirrored> will be added to the volume letter.

   
   

   
     [Figure] Shared Disk DR Resource Added

   
   

4. Click the 'Additional Setup' to define the synch mode and the disk test option.
   

   Info
   <Sync mode> Once selected, the sync mode cannot be reversed. So, be careful. Sync mode : Response is slow but there is no data loss involved. Async mode : Relatively faster than the sync mode but it can suffer from data loss. <Disk test option> Set the disk test command option. For more details on the function, refer to MSDN.

    
    [Figure] Shared Disk DR Additional Setting

   
   

5. Press the Finish button after adding the shared disk DR resource, the following warning screen will be displayed.
   Check again if it is the correct source server selected.
   If not, then click on the 'Cancel' button and set it again. 

   
   [Figure] The warning screen displayed when you press the Finish button after adding the shared disk DR resource   
    

6. Click the 'Finish' button to add the shared disk DR resource. You can immediately check the result in the management web console.

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