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.

Deleting

Select resource type → right click → delete resource.

State

The following table explains the state of SCSI lock resource.

State	Agent command	Description
Online Key is exclusively in reservation state in online node. Key cannot be registered in other node.	Offline	It changes to offline state while maintaining the reservation.
	Monitoring	Check if there is a reservation made on the disk.
Offline 1. Disk is reserved but the shared disk resource is offline. 2. It is offline state due to standby node have taken the key.	Online	Register a key for a disk reservation and wait for a response from the other node.
	Monitoring	Check if there is a reservation made on the disk.
Fault It occurs when a reservation is taken away when it is online.	Online	Register a key for a disk reservation and wait for a response from the other node.
	Offline	It changes to offline state while maintaining the reservation.

Attribute

Display the specified attribute of SCSI lock resource.

Attribute	Description	Type	Default	Edit	Note
DiskDUID	DUID(Device Unique ID, 16 byte as Hex value) of disk is used as identifier. Specify DUID of reserve disk. This value is secure when DriveLetter atribute us specified. Try not to insert this value yourself. DUID information can be checked through scsicmd.cmd command and both nodes need to have the same value.	STRING		X
DriveLetter	Disk to be used for reservation is specified by drive letter. When register drive letter, DUID of connected disk will automatically obtain in drive letter.	STRING		X
Key	MAC address is used as Reserved-Key. This key is automatically set by agent.	STRING
PreemptWaitCount	Time waiting for permission of other node before reservation Waiting time is (PreemptWaitCount * MonitorInterval) seconds.	INTEGER	2 times	O
PrType	This attribute decides on how to give the permission of reserve disk access as Persist Type specified in SCSI3-PR spec. There are 6 types but MCCS uses 5 (Write Exclusive Registor Only) .	INTEGER	Prtype=5	O
Fault	SCSI 디스크가 정상인지 비정상인지 나타냅니다.	STRING	false	X

scsicmd.cmd Command

Syntax

 scsicmd [option] [diskID]

Option

This is the utility to use SCSI 3 PR command more easily. Following is the provided options

Option	Description
-c diskId	It deletes Reserved-Key and Registered-Key of local node of disk.
-cf diskId	It forcely deletes Reserved-Key and Registered-Key of local node of disk. This is mostly used when access to disk is inavailable.
-r diskId	Registers diskld-specified disk.
-l diskId	If node is yet to reserve, close the specified disk and reserve.
-lf diskId	It forcelu closes specified disk and reserve, even node has been reserved. Meaning, it takes over other reservation.
-h	It shows help of scsicmd.
no option	It shows the state information regarding disk reservation when there is no other option.

Print Sample

The output of 'scsicmd' command from source node (C29043F13 is MAC Address of NIC from source node.)

Disk	DUID(DeviceUniqueID)	Letter	PR	Reserved-Key	Registered-Keys
0		C	No	none	none
1		D	No	none	none
2	00c0ffd796550000f38be34f01000000	E	Yes	none	none
3	00c0ffd796550000138ce34f01000000	F	Yes	C29043F13*	C29043F13*
4	00c0ffd7965500002e8ce34f01000000	G	Yes	none	none

The output of 'scsicmd' command from target node (C29043F13 is MAC Address of NIC from source node.)

Disk	DUID(DeviceUniqueID)	Letter	PR	Reserved-Key	Registered-Keys
0		C	No	none	none
1		D	No	none	none
2	00c0ffd796550000f38be34f01000000	E	Yes	none	none
3	00c0ffd796550000138ce34f01000000	F	Yes	C29043F13	C29043F13
4	00c0ffd7965500002e8ce34f01000000	G	Yes	none	none

Disk 0 (C:)'s PR is set to "No". So, you can see that the disk does not support SCSI3-PR. (In general, the "C" drive is the system booting disk and it does not support SCSI.)
Besides, Disk 1 (D:)'s PR is set to "No". You can see that the disk does not support SCSI3-PR.(Internal Disk does not support SCSI3-PR.)
Disk 2 (E:) and Disk 4 (G:)'s SCSI3-PR is set to "Yes". So, it means that the storage used supports SCSI3- PR. Their Reserved-Key and Registered-Keys are set to "none". It means that they are not yet to be registered.
Disk 3 (F:)'s Reserved-Key and Registered-Keys at the source node are set to C29043F13* and C29043F13* and at the target node to C29043F13 and C29043F13. This means that a SCSI reservation agent was used to create the resource.
The (*) marked node with the Reserved-Key and Registered-Keys indicates that a disk is reserved. In addition, only the * marked node with the Reserved-Key value (ex: C29043F13*) can access the disk.

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7.9 SCSI Lock

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