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SCSI reservation agent can manage and protect the shared disks from the H/W perspectives.
If you use a SCSI reservation agent, you can safely protect the data disk against the split brain situations caused by external elements such as OS hanging.
If the shared storage supports SCSI3-PR (Persistent Reservation), you can use it together with SCSI reservation agent to improve the level of security for the shared data.
If you want to access the storage from systems (many or one) through SCSI3-PR, the system must have a storage reserved key. If not, storage access will not be granted.

* Note - If the storage does not support SCSI3-PR (Persistent Reservation), it cannot use a SCSI-reservation agent.

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SCSI-Lock resource can be configured in 2 ways with shared disk resource. Configuration process changes according to the attribute value of SCSI Lock resource.


1. When shared disk and SCSI Lock resource is configured with the same LUN (PrType=1)

When you define an attribute of SCSI Lock resource as PrType=1, you can configure shared disk and SCSI Lock resource with the same LUN.
SCSI Lock agent manages DATA-DISK(shared disk resource) using by MOUNT/UMOUNT command and manages LOCK-DISK(SCSI Lock resource) using by SCSI-3 PR as shown below.
By setting up the dependency between SCSI Lock resources and shared disk resources, any damage of data-disk can be prevented which may occur in such case like Split-Brain.

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[Figure] Shared disk and SCSI Lock resource configured in same LUN (PrType=1)

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At this type, shared disk agent manages DATA-DISK where user data is stored and SCSI lock agent uses exclusive LUN for key reservation. Therefore, two LUNs is required at least.
Lock-disk volume is created for 1:1 dependency with shared disk agent like a figure below.
By using multiple shared disk agents, and one SCSI reservation agent, you can configure the dependency relationship for use.
The storage capacity of this disk can be small because it is used to check the node that can access the storage.
By setting the dependency between the SCSI reservation resource and the shared disk resource, you can prevent damages to DATA-DISK caused by Split-Brains and so on. 

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[Figure] Shared disk and SCSI Lock resource is configured with different LUN( PrType=3)

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