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Veritas Cluster Server I/O Fencing Deployment Considerations

Created: 25 May 2011 • Updated: 23 Oct 2013
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Executive Summary
I/O Fencing provides protection against data corruption and can guarantee data consistency in a clustered environment. Data is the most valuable component in today’s enterprises. Having data protected and therefore consistent at all times is a number one priority.

This White Paper describes the different deployment methods and strategies available for I/O Fencing. It is designed to illustrate configuration options and provide examples where they are appropriate.

Symantec has led the way in solving the potential data corruption issues that are associated with clusters. We have developed and adopted industry standards (SCSI-3 Persistent Group Reservations [PGR]) that leverage modern disk-array controllers that integrate tightly into the overall cluster communications framework.

Background
Clustering, in its nature, exposes a risk of data corruption, due to the fact that independent nodes have access to the same data. In its infancy, this technology caused data corruptions. As clustering evolved, different technologies have been developed to prevent data corruption.

In short, the problem arises when two nodes are accessing the same data, independently of each other. This is termed “split brain” and is outlined in the “Introduction” chapter of this document (next page).
Preventing data corruption during a split brain scenario is relatively easy. However, some cluster solutions handle this situation by forcing downtime to the applications. This is not acceptable for today’s demanding environments.

As VCS evolved, several methods of avoiding split brain and data corruption have been put into place. Since VCS 3.5 (Released in 2001), a feature named I/O Fencing has been available. I/O Fencing can eliminate the risk of data corruption in a split brain scenario.

This document focuses on the various options for I/O Fencing, implementation considerations and guidelines. It provides a comparison between the different deployment methods.
NOTE: VCS for Windows (known as Storage Foundation for Windows HA) uses another method to prevent data corruptions in split brain scenarios. Please refer to public documentation for the Storage Foundation for Windows HA release for more information.

Written by: Niclas Blaback, Technical Product Manager VCS, Anthony Herr, Technical Product Manager VCS

Download the complete PDF. Contents include:

EXECUTIVE SUMMARY
THIRD-PARTY LEGAL NOTICES
LICENSING AND REGISTRATION
TECHNICAL SUPPORT
SCOPE OF DOCUMENT
AUDIENCE
BACKGROUND
INTRODUCTION
INTRODUCTION TO VCS TOPOLOGY
SPLIT BRAIN OUTLINED – WHAT IS THE PROBLEM?
  There are three types of split brain conditions:
  Regular split brain
  Serial Split brain
  Wide Area Split brain
WHAT ARE THE MOST COMMON CASES FOR A SPLIT BRAIN TO HAPPEN?
GENERAL NOTES OF I/O FENCING
MEMBERSHIP ARBITRATION
DATA PROTECTION
  SCSI3 Persistent Reservations for Private DiskGroups:
  SCSI3 Persistent Group Reservations Shared DiskGroups (CVM/CFS)
AGENTS RELATED TO I/O FENCING
  DiskGroup Agent notes and attributes
  MonitorReservation (Boolean-scalar)
  Reservation
  CoordPoint Agent notes
I/O FENCING CAN BE ENABLED FOR ALL ENVIRONMENTS
NON SCSI3 BASED FENCING
PREFERRED FENCING
DEPLOYING I/O FENCING
WORKFLOW – HOW TO DEPLOY I/O FENCING IN YOUR ENVIRONMENT.
CHOOSING COORDINATION POINT TECHNOLOGY
  Disk-based coordination points
  CP Server based coordination points
  A combination of CP Servers and Coordinator using SCSI3-PR
CHOOSING COORDINATION POINT PLACEMENT
DEPLOYING I/O FENCING
DEPLOYING PREFERRED FENCING (OPTIONAL)
CP SERVER CONSIDERATIONS
  CP Server scalability requirements
  Clustering the CP-Server itself
I/O FENCING DEPLOYMENT SCENARIOS
SCENARIO 1: ALL NODES IN THE SAME DATA CENTER USING DISK BASED COORDINATION POINTS.
SCENARIO 2: ALL CLUSTER NODES IN THE SAME DATACENTER, WHILE REDUCING THE AMOUNT OF STORAGE USED FOR COORDINATOR DISKS
SCENARIO 3: CAMPUS CLUSTER CONFIGURATION USING THREE SITES
SCENARIO 4: REPLACING ALL COORDINATION DISKS WITH CP SERVERS – AVAILABILITY
SCENARIO 5: REPLACING ALL COORDINATION DISKS WITH CP SERVERS – FLEXIBILITY
COORDINATION POINTS AVAILABILITY CONSIDERATIONS