Booting from SANs and Using SANs with Clusters

Windows Server 2008 supports booting from a SAN, having multiple clusters attached to the same SAN, and having a mix of clusters and stand-alone servers attached to the same SAN. To boot from a SAN, the external storage devices and the host bus adapters of each server must be configured appropriately to allow booting from the SAN.

When multiple servers must boot from the same external storage device, you must either configure the SAN in a switched environment or you must directly attach it from each host to one of the storage subsystem's Fibre Channel ports. A switched or direct-toport environment allows the servers to be separate from each other, which is essential for booting from a SAN.

The use of a Fibre Channel-Arbitrated Loop (FC-AL) configuration is not supported because hubs typically don't allow the servers on the SAN to be isolated properly from each other-and the same is true when you have multiple clusters attached to the same SAN or a mix of clusters and stand-alone servers attached to the same SAN.

Each server on the SAN must have exclusive access to the logical disk from which it is booting, and no other server on the SAN should be able to detect or access that logical disk. For multiple-cluster installations, the SAN must be configured so that a set of cluster disks is accessible only by one cluster and is completely hidden from the rest of the clusters. By default, Windows Server 2008 will attach and mount every logical disk that it detects when the host bus adapter driver loads, and if multiple servers mount the same disk, the file system can be damaged.

Detecting SAN configuration problems

On an improperly configured SAN, multiple hosts are able to access the same logical disks. This isn't what you want to happen, but it does happen and you might be able to detect this configuration problem when you are working with the logical disks. Try using Windows Explorer from multiple hosts to access the logical disks on the SAN. If you try to access a logical disk and receive an "Access Denied," "Device Not Ready," or a similar error message, this can be an indicator that another server has access to the logical disk you are attempting to use. You might see another indicator of an improperly configured SAN when you add or configure logical disks. If you notice that multiple servers report that they've found new hardware when adding or configuring logical disks, there is a configuration problem with the SAN. If there is a configuration problem with clusters, you can see the following error events in the System logs:

• Warning event ID 11 with event source %HBADriverName%, "The driver detected a controller error on Device\ScsiPortN."
• Warning event ID 50 with event source Disk, "The system was attempting to transfer file data from buffers to \Device\HarddiskVolumeN. The write operation failed, and only some of the data may have been written to the file."
• Warning event ID 51 with event source FTDISK, "An error was detected on device during a paging operation."
• Warning event ID 9 with event source %HBADriverName%, "Lost Delayed Write Data: The device, \Device\ScsiPortN, did not respond within the timeout period."
• Warning event ID 26 with event source Application Popup, "Windows-Delayed Write Failed: Windows was unable to save all the data for the file \Device\HarddiskVolumeN\ MFT$. The data has been lost. This error may be caused by a failure of your computer hardware or network connection. Please try to save this file elsewhere."

To prevent file system damage, the SAN must be configured in such a way that only one server can access a particular logical disk at a time. You can configure disks for exclusive access using a type of logical unit number (LUN) management such as LUN masking, LUN zoning, or a preferred combination of these techniques. With Windows Server 2008, you can use the Storage Manager For SANs console to manage Fibre Channel and iSCSI SANs that support the Virtual Disk Service (VDS) and have a configured VDS hardware provider. Before you can use this console, you must use the Add Features Wizard to add the Storage Manager For SANs feature to the server. After you add this feature, Storage Manager For SANs is available as an option on the Administrative Tools menu.