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Oracle10g Grid Computing with RAC
Chapter 3 - High Performance and Highly Available
Database Clusters

I/O Fencing – exclusion strategy

For example, in Sun Cluster, disk fencing is done through SCSI-2 reservation for dual hosted SCSI devices and for multi-hosted environment through SCSI-3 PR. Veritas Advance Cluster uses the SCSI-3 persistent reservation to perform IO fencing. In case of Linux clusters, CFS like Polyserve and Sistina GFS are able to perform IO fencing by using different methods like fabric fencing that uses SAN access control mechanism.

SCSI-3 PR

SCSI-3 PR (persistent reservation) supports multiple nodes accessing a device while at same time blocking access to other nodes. SCSI-3 PR reservations are persistent across SCSI bus resets or node reboots and it also supports multiple paths from host to disk. For SCSI-2 disks, reservations are not persistent—they do not survive node reboots.

SCSI-3 PR uses a concept of registration and reservation. Systems that participate, register a ‘key’ with SCSI-3 device. Each system registers its own key. Then registered systems can establish a reservation. With this method, blocking write access is as simple as removing registration from a device. A system wishing to eject another system issues a ‘pre-empt and abort’ command and that ejects another node. Once a node is ejected, it has no key registered so that it cannot eject others. This method effectively avoids the split-brain condition.

Another benefit of the SCSI-3 PR method is that since a node registers the same key down each path, ejecting a single key blocks all IO paths from that node. For example, SCSI-3 PR is implemented by EMC Symmetrix, Sun T3 and Hitachi Storage systems. In case of SCSI-2 reservation, it works only with one path with one host.


For more information, see the book Oracle 11g Grid and Real Application Clusters 30% off if you buy it directly from Rampant TechPress . 

Written by top Oracle experts, this RAC book has a complete online code depot with ready to use RAC scripts.


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Burleson Oracle Consulting offers Senior Oracle consultants which are available for all areas of Oracle support.  Common Oracle consulting support activities include short-term Oracle tuning, Oracle database troubleshooting, Oracle9i and Oracle10g migration, Oracle9iAS App Server consulting, Oracle design reviews and Oracle requirements evaluation support. Oracle support and Oracle consulting services are priced by the hour, so you only pay for what you need. These one-time Oracle consulting services commonly include:

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