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Figure 26-3 Controller caches in front of disk sharing.
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Other solutions to the shared-disk problem involve RAID striping and caching in the disk system itself. In this configuration, the cache is shared by all nodes, and we say that the cache is behind the sharing, as shown in Figure 26-4. Here, the striping mechanisms and the cache are viewed identically by all of the controllers in the system, and both read caching and write caching are safe.
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Failover Clustering Installation and Configuration
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Disk controller Cache
Disk controller
Figure 26-4 Controller cache behind disk sharing.
Fibre Channel and iSCSI disk subsystems allow the RAID controller to be in the disk enclosure, rather than in the computer system. These systems offer good performance and fault tolerance. In fact, many RAID systems of this type offer fully redundant controllers and caches. Many of the newer RAID systems use this type of architecture. Let s look at some disk subsystems in detail:
I/O Subsystems As mentioned, various types of I/O subsystems support clustering. The three main types of I/O subsystems are as follows:
This is a SCSI system with multiple initiators (controllers) on a SCSI bus that address JBOD (short for just a bunch of disks ). In this setup, the disks are individually addressed and must be either configured into a stripe using Windows 2000 striping or addressed individually. This subsystem is not recommended.
SCSI JBOD Internal RAID A RAID controller is used in each server. The disadvantage of this subsystem is that the RAID logic is on the board that goes in the server and, thus, the controller caches must be disabled. External RAID The RAID controller is shared by the systems in the cluster. The cache and the RAID logic are in the disk enclosure, and a simple host bus adapter (HBA) is used to communicate with the external controller. External RAID can be implemented either via a Storage Area Network (SAN) or Network Attached Storage (NAS) that includes iSCSI.
SAN The Storage Area Network (SAN) is an ideal platform for clustering because of its robustness as well as the redundancy typically built into a SAN. In addition, SAN storage typically has significant capacity and is high performance. iSCSI The iSCSI storage subsystem is a new technology that uses the SCSI protocol encapsulated in an IP packet. iSCSI provides the flexibility and cost benefit of network storage while providing a robust and efficient transport layer that supports clustering.
Part VI
High Availability
The next two sections address only the two RAID solutions. The SCSI JBOD solution is not advisable unless the cluster is small and cost is a major issue. Internal RAID Internal RAID controllers are designed such that the hardware that controls the RAID processing and the cache reside in the host system. With internal RAID, the shared disk system is shared behind the RAID striping, as shown in Figure 26-5.
Server A Server interconnect
Server B
RAID controller
RAID controller
Shared disks
Figure 26-5 Internal RAID controller.
Because the cache is located on the controller, which is not shared, any data in the cache when the system fails will not be accessible. This is a big problem when a relational database management system (RDBMS) is involved. When SQL Server writes data to disk, that data has been recorded in the transaction log as having been written. When SQL Server attempts to recover from a system failure, these data blocks will not be recovered because SQL Server thinks that they have already been written to disk. In the event of a failure in this type of configuration, the database will become corrupted. Therefore, vendors certify their caching RAID controllers for use in a cluster by disabling the cache (or at least the write cache). If the cache has been disabled, SQL Server is not signaled that a write operation has been completed until the data has actually been written to disk. Note SQL Server performs all writes to disk in a nonbuffered, noncached mode. Regardless of how much file system cache is available, SQL Server will not use it. SQL Server completely bypasses the file system cache, as do most RDBMS products. In certain situations, using the controller cache can provide a great performance benefit. This is particularly true when you are using a RAID-10 (aka RAID-0/1 or -1/0) or RAID5 configuration because writes incur additional overhead with these RAID levels. To use
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