The shared disk DR resource replicate a shared disk volume of a cluster to an offsite node of disaster recovery purpose.

The shared disk DR resource is composed of three nodes that manage the shared volume. MCCS is only operable at the server that composes the shared volume.
In other words, MCCS is not involved with operation of the offsite node, so, administrators need to perform manual operation or register necessary actions for processing in advance.

Data is replicated from the source-primary node to the offsite target node and the source-secondary node that cannot access the shared volume recognizes the shared volume as the one having the "NONE" role.
Thus, the offsite target node and the "NONE" node are locked and you cannot write or read to and from them. The goal is to prohibit the use of the corresponding disk volume of the node so that the data integrity can be guaranteed.

Major content of this page

[Figure] Mirroring Configuration

Data Mirror Mode

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

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.

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.

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

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

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

Delete

Select the resource -> Right click with the mouse -> Delete the resource.
The deleted resource will immediately disappear from the management web console.

Status

The following table explains the status changing of the MCCS resource caused by a user's command and the status.
The command assumes that it is generated by a user.

Shared disk DR agent: Manages the mirror disk. Copying program should be installed.

Status	Agent's functions	Description	Note
Online In this status, you can access the source volume and perform a writing test properly.	Offline	Regardless of the mirror role, the LOCKVOLUME command is used to lock the volume. If the mirror volume is not defined, the LOCKVOLUME command is ignored and regarded as a failure.
	Monitoring	Before monitoring, update the status of resource first. Perform a disk write test to determine the status of resource. If the writing test is successful, it is regarded as the online state. If the writing test failed on the disk, it is regarded as the offline state.
Offline Except for the online and trouble states, it is always offline.	Online	Type of operation is determined by the role of the mirror volume at the start node. <Source volume> 1. You can unlock and write the volume. 2.To unlock, you can use the 'UNLOCKVOLUME' menu in the copy program. Check whether there are dirty bits. <Target or NONE volume> 1. If the mirror role is target or NONE volume, run the SWITCHOVERVOLUME command.
	Monitoring	Refer to the description of monitoring as above.
Trouble If a writing test failed online, or an attempt to go online is failed, the trouble state is displayed. *Failover deactivation state When a mirror network communication failure, target node or target disk failure occurs, the mirror state is changed from 'MIRRORING' to other, such as 'MIRROR_PAUSED'. If the mirror status is not 'MIRRORING', a failover will take place and it can cause data losses or damages. To prevent this problem, an agent deactivates the failover mode if the mirror status is not "MIRRORING". If the failover mode is deactivated, no failover will take place even if manual failover or troubles occur. If mirror network communication failures are recovered, and communication resumes, it will automatically perform RESYNC. When the mirror status is changed to MIRRORING, the failover mode will be also automatically activated.	Online	Refer to the above online command.
	Offline	Regardless of the mirror role, the LOCKVOLUME command is used to lock the volume. If the mirror volume is not defined, the LOCKVOLUME command is ignored and regarded as a failure.

Attribute

Show the unique attribute of the shared disk DR resource.

ArgList Factor	Description	Type	Default	Modification	Note
ChkDsk	When the volume is mounted, according to the 'Disk Test Options', disk is tested for errors. If it is set to true, and if dirty bits are set to the online volume, the ChkdskArg option will be used to run checkdisk. Before running checkdisk, the mirror copying will be automatically stopped and resynchronization will be performed after finished.	BOOLEAN	false	O
ChkdskArg	When the checkdisk (Chkdsk) option is set to true, the check disk command option is activated.	STRING	/F	O
DirtyBit	It means that dirty bits are set to the target mirror volume. In general, if you need to check the integrity of the target volume, the system will set this.	BOOLEAN	false	X
DriveLetter	The mirror volume is assigned to the drive character. It should be configured to obtain the same value at the source and the target systems.	STRING		X
DRState	The mirror status of the mirror volume. It is renewed from an event log.	STRING		X
DRType	Determine the mirror synchronization mode. Set it to A or S. ( A - async mode, S - sync mode) * If it is set to asynch mode, data loss can occur.	STRING	A	X
ResyncNeed	It represents the total sum of the blocks requiring re-synchronizing.	INTEGER		X
ResyncTotal	It is the total sum of the re-synchronizing blocks.	INTEGER		X
SharedSrcIp	It is the IP address used to manage the volume shared between two nodes.	STRING		X
SharedSrcRole	It represents the role of the shared volume for each node.	STRING		X
TargetDRIp	The IP address used to copy the volume.	IP ADDRESS		X
TargetDRLetter	It is the drive character for the DR target volume.	STRING		X
TargetDRRole	It represents the role of the shared volume for the target DR node.	STRING		X
TotalSpace	It represents the total size of the target mirror volume.	INTEGER		X
UsedSpace	It represents the available size of the target mirror volume.	INTEGER		X

Attribute value of ArgList arguments are the values that are added resources and the value can be changed or viewed through ’Specified Attribute’ list on the detailed information panel.

MCCS 4.2 English Online Help

7.11 Shared Disk DR