Shared Disks in VS .NET

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Shared Disks
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Shared disks are the disks that contain the critical data. Both systems need physical access to these disks, although it is critical that only one system at a time access them. If both servers try to write to shared disks at the same time without specialized software in place, data corruption is virtually inevitable. If one system simply tries to read the disks while the other writes, the reader will likely run into problems when the data it is reading changes unexpectedly. Access to shared disks must be limited to only one node at a time unless specialized software that specifically changes these rules is in place. When you share disks, you are actually sharing data. There is more than one way to share data among the nodes in a cluster. The most common and preferable way is called dual hosting. In this model, both servers are physically connected to the same disks at the same time. Access is arbitrated by external software that runs on both servers. When a failover occurs and access to the shared disks migrates from one server to the other, all the data successfully written by one server is guaranteed to be accessible by the other.
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Local Clustering and Failover
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The other method of sharing data is through a technology called shared nothing. In this model, data is replicated across a network (usually either the heartbeat network, or another parallel private network) between the servers. Shared nothing is a much more complicated model, since it requires a functional network and a functional host on the other side to ensure that the writes actually succeed. Another significant issue with shared nothing clusters is as follows. When server agony is healthy and functional in a shared nothing configuration, it replicates data to ecstasy. When agony fails, it fails over to ecstasy. While ecstasy operates, agony may not be functioning, and so agony will not be able to keep current on updates to ecstasy. When agony is repaired and is ready to go back into service, additional work will be required to refresh its data to match ecstasy. Depending on how much data is involved, it could take a long time, and a tremendous amount of effort and network bandwidth.3 If ecstasy were to fail during that refresh period, agony could not take over. What s far worse, though, is that if an event occurred that brought both agony and ecstasy down a power outage, for example after ecstasy had taken over. When the power was restored to both nodes, neither would know which node was running as the primary before, and therefore, neither would know that agony s data was out of date. If agony came up as the primary, it would begin replicating data to ecstasy immediately, and the result would be two hosts with two different sets of data, neither set complete or correct. Dual hosting is a superior method for sharing disk data, although it requires specific hardware that permits this dual hosting. Not all disk or controller hardware can handle the dual hosting of SCSI-based disks and arrays, especially in the Windows world. Check with your vendors before you make blind assumptions. SANs are another way to achieve dual-hosted storage. For more on SANs, please refer to 8, SAN, NAS, and Virtualization. When configuring disks for use on critical systems in a cluster, it is important to remember the availability enhancing techniques that we discussed in 7, including mirroring, RAID, and multipathing.
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Disk Caches
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A disk cache is a chunk of memory that is used to hold disk data before it is written to disk, or to hold data that is read from the disk before it is actually needed. Since memory access can be 1,000 times (or more) faster than disk, there are tremendous performance implications by using disk cache. From an availability perspective, prereading (or prefetching) disk data has no implications. However, writing data to the cache that never makes it to disk can have significant availability implications.
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3 This is only the case for software- or host-based replication and does not apply if hardware replication is used, as hardware-based replication can update its remote side even if the host has gone down. For more on the different types of replication, see 18, Data Replication.
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