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[Linux-cluster] fence_scsi - restrictions in use


The man page that I have for fence_scsi(8) lists (among others) the following restrictions in use:

- The fence_scsi fencing agent requires a minimum of three nodes in the cluster to operate.
- In addition, fence_scsi cannot be used in conjunction with qdisk.

I am puzzled by those restrictions, as I remember an ancient Digital/Compaq TruCluster V1.x that also used SCSI reservations and had no such restrictions.

I cannot see a logical linkage between the SCSI registration and node count in the cluster. Each node does its own reservation attempts and registrations. 

Question 1:
Why a minimum of 3 nodes are required?

I cannot also see any linkage between SCSI reservations and use of quorum disk. SCSI reservations are done on a per device (LUN) basis. Obviously, the qdisk should be excluded from this process, I believe. But once this is done there should not be a problem.

Question 2:
Why qdisk cannot be used in conjunction with fence_scsi?

SCSI reservations are such a powerful mechanism. Apart from use for fencing they could be used to enforce exclusive access to non-clustered filesystems only to a node that started the service using this particular filesystem. 

Question 3:
Does Linux cluster use SCSI reservations to enforce exclusin access to non-clustered filesystems on shared storage?

Thanks and regards,

Chris Jankowski

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