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Chapter 8. Configuring Fencing using SCSI Persistent Reservations
This chapter provides the procedures for configuring fencing using SCSI persistent reservations in a Red Hat cluster using the Conga configuration tool.
SCSI persistent reservations provide the capability to control the access of each node to shared storage devices. Red Hat Cluster Suite employs SCSI persistent reservations as a fencing methods through the use of the
fence_scsi agent. The fence_scsi agent provides a method to revoke access to shared storage devices, provided that the storage support SCSI persistent reservations.
Using SCSI reservations as a fencing method is different from traditional power fencing methods. It is important to understand the software, hardware, and configuration requirements prior to using SCSI persistent reservations as a fencing method.
8.1. Technical Overview of SCSI Persistent Reservations
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In order to understand how Red Hat Cluster Suite is able to use SCSI persistent reservations as a fencing method, it is helpful to have some basic knowledge of SCSI persistent reservations.
There are two important concepts withing SCSI persistent reservations that should be made clear: registrations and reservations.
8.1.1. SCSI Registrations
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A registration occurs when a node registers a unique key with a device. A device can have many registrations. For our purposes, each node will create a registration on each device.
8.1.2. SCSI Technical Overview
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A reservation dictates how a device can be accessed. In contrast to registrations, there can be only one reservation on a device at any time. The node that holds the reservation is know as the "reservation holder". The reservation defines how other nodes may access the device. For example, Red Hat Cluster Suite uses a "Write Exclusive, Registrants Only" reservation. This type of reservation indicates that only nodes that have registered with that device may write to the device.
8.1.3. SCSI Fencing with Persistent Reservations
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Red Hat Cluster Suite is able to perform fencing via SCSI persistent reservations by simply removing a node's registration key from all devices. When a node failure occurs, the
fence_scsi agent will remove the failed node's key from all devices, thus preventing it from being able to write to those devices.
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