SCSI Lock uses SAN protocol which is called SCSI-3 PR(Persistent Reservation). It is essential to equip SCSI-3 PR(Persistent Reservation) storage that to use SCSI Lock resource. SCSI-3 PR supports disk access from a singe node and multiple nodes. And also it blocks access to the disk from other nodes.

When a disk volume is managed by this agent, only one node can access that. SCSI reservations are needed to prevent any data corruption in environment where disk volumes are shared between nodes.

* Note - If the storage does not support SCSI3-PR (Persistent Reservation), it cannot be used.

Table of Contents

How to work

Disk resource added through SCSI Lock has registration key for each node and it can be checked by it's attribute. Online node sets its Registered-key as Reserved-Key.

Only the node that has imported the disk with this key can access to the disk.

By setting SCSI Lock resource's dependency with shared disk resource, exclusive access to data is protected in split-brain situation.

When split-brain is occurred, standby node also attempt to bring online a shared disk during active node keep online state. But standby node cannot register the Reserved-Key if active node dose not release this and it gives up to bring online the resources.

Requirement

External storage must support SCSI-3 PR.
Volume should be configured by LUN(Logical Unit Number) unit. Partitioned volume in a LUN is not supported.

Types of Shared disk resource

To configure HA with shared storage environment, MCCS provides shared disk agent and SCSI Lock agent.
You can use only shared disk resource, or use with SCSI Lock resource. Shared disk resource depends on SCSI Lock resource and multiple shared disk resource can depends on a SCSI Lock resource.

An environment where only shared disk agent is used

The disk volume is partitioned in a LUN.
External storage does not support SCSI-3 PR.

Difference between shared disk agent and SCSI Lock agent
1. Shared disk agent manage disk access in local node, but SCSI Lock agent manage node access in storage.

Disk management function in software perspective (shared disk agent)

Shared disk agent manage access control of shared disk through Mantech Volume Lock (MVL) kernel driver.
Data will be corrupted if writing access is done from multiple nodes in shared disk. MVL locks access to the shared disk resource. You cannot access the volume of shared disk before it unlocks access. Only a node allow to access to the shared disk resource by this.
MVL can be managed both in LUN unit and partition unit.

Disk management function in hardware perspective (SCSI Lock agent)

SCSI Lock agent manage Write control and access control of LUN by using SAN protocol.
Only the node that has imported the disk with this key can access to the disk.

2. Both the shared disk agent and the SCSI reservation agent can read and write data.
However, a SCSI reservation agent intends to protect the shared storage resource, so you are not recommended to use it alone to read and write data.

Configuration

MCCS uses the MVL driver to manage DATA-DISK(Share disk) and uses the SCSI3-PR function to manage LOCK-DISK(SCSI lock).
By setting the dependency between the SCSI lock resource and the shared disk resource, you can prevent damages to DATA-DISK caused by Split-Brains and so on.

[Figure] Shared disk and SCSI Lock resource configured in same LUN

DATA-DISK

It is a shared disk resource and contains application datas.

LOCK-DISK

It is SCSI lock resource and contains reservation key.

Configuration Sample

[Figure] Shared disk and SCSI Lock resource configured in same LUN

First, register shared disk agent according to the procedure and after check if it works normally, check disk of SCSI Lock agent.

Registration Procedures

Install MCCS on both nodes
Set group
Add shared disk resource
Enable shared disk resource and check offline on both nodes
Add SCSI Lock resource (Select drive letter that created shared disk resource)
Enable SCSI Lock resource and check key in resource attribute of MCCS console.
Configure dependency of resources (SCSI Lock resource must go online first)
Test failover from node 2 to 1
Test failover from node 1 to 2
Set application that customer uses

Adding

Add the SCSI lock resource to a group.

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.
Select 'SCSILock' from Resource Type lists and click 'Next' button. When add a resource from the SCSILock resource type name, this step will be skipped.
Enter the resource name. If you skip enter resource name, this will be given 'SLock_value' which value is selected driver letter.
Select the driver letter.

[Figure] Adding SCSI lock resource
Click 'Finish' button to add the SCSI lock resource.