Windows Server 2008 Failover Clustering - Storage Changes
In a previous blog, I talked a little bit about some of the major changes to disk storage for Windows Server 2008 Failover Clustering. Now, while I wait for dinner to cook, is a good time to cover some of the changes.
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2TB Limit- The biggest change, in my opinion, has to be elimination of the Master Boot Record (MBR) requirement for clustered disks. MBR forced a limitation to the physical disk resource size of 2TB. Now, Windows Server 2008 supports the use of GUID (sometimes called Global) Partition Table (GPT) which allows up to 16 Exabyte. The practical limit is really around 160-200 TB based on today's technology. Keep in mind that just because you can do it doesn't mean you should do it. Can you imagine how long it would take to degrag a 100TB disk? How about reindexing it? How about chkdsk?
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SCSI Bus Resets - In Windows Server 2003 Server Clustering, SCSI bus resets are used to break disk reservations forcing it to become disconnected so that another controller can take control of the disks. The problem with SCSI bus resets is that they require all devices on the same bus to lose their connection. These resets were not exactly warmly received by the disks that were impacted without any reason for it. In Windows Server 2008 Failover Clustering, SCSI bus resets are no longer used as persistent reservations are now required.
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Persistent Reservations - WIndows Server 2008 Failover Clustering supports the use of persistent reservations. This means that directly attached SCSI storage will no longer be supported in 2008 for Failover Clustering. Serially Attached Storage (SAS), Fiber Channel, and iSCSI will be the only supported technology. However, not all vendors support persistent reservations, so this will be a problem as organizations move to Windows Server 2008 Failover Clustering without full and proper testing.
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Maintenance Mode - In maintenance mode allows administrators to gain exclusive control to clustered disks
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Disk Signatures - In Windows Server 2003 (and earlier), administrators would come to our classes at
http://www.clusterhelp.com/ and would cringe whenver we talked about disk signatures. I even had one burst into tears from reliving past stress. OK, maybe not real tears. Anyways, clustering used disk signatures to identify each clustered disk. The disk signature, at sector 0, is often an issue in disaster recovery scenarios (in class I show how to totally get around disk signature issues). To make it easier, Failover Clustering makes use of SCSI Inquiry Data is often written to a LUN by a SAN. In the event the disk signature gets out of whack, the disk signature can be reset once the disk has been verified by the SCSI Inquiry Data. If for some reason, both the disk signature and the SCSI Inquiry Data are not available or are misconfigured/corrupted, an administrator can a Repair button is available on the Physical Disk Resource properties general page. The Repair button can be used to point the actual disk to the resource.
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Disk Management - The virtual disk service (VDS) APIs first became available with 2003 R2. Using the disk tools in Windows Server 2003 R2 (and now 2008), an administrator can build, delete, and extend volumes in the SAN.
All in all, there have been some pretty significant changes when it comes to the way Windows Server 2008 Failover Clusters work with disk storage.
