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  Oracle Tips by Burleson

Oracle RAC Cache Fusion and I/O Bandwidth

Oracle RAC Cache Fusion uses a high-speed IPC interconnect to provide cache-to-cache trasfers of data blocks between instances in a cluster. This is called data block shipping. This eliminates the disk I/O and optimizes read/write concurrency. Block reads take advantage of the speed of IPC and an interconnecting network.

The cache-to-cache data transfer is performed through the high speed IPC interconnect . The Oracle Global Cache Service(GCS) tracks blocks that were shipped to other instances by retaining block copies in memory. Each such copy is called a past image (PI). The GCS, via the LMSx background process, tracks one or more past image versions (PI) for a block in addition to the traditional GCS resource roles and modes. In the event of a node failure, Oracle can reconstruct the current version of a block by using a saved PI.

Simultaneous Reads on Different RAC Nodes

Simultaneous reads of the same data block on multiple nodes cause bandwidth bottlenecks, but they can also cause cache conflicts within RAC. Real Application Clustersresolves this situation because multiple instances share the same blocks for read access without cache conflicts. Conflicts can only occur for read/write and write/write situations.

Simultaneous Reads and Writes on Different RAC Nodes

Simultaneous reads and writes on different nodes are the dominant form of concurrency problems in Online Transaction Processing (OLTP), hybrid OLTP, and data warehouse applications. A read of a data block that has been modified recently can be for either the current version of the block or for a read-consistent previous version. In both cases, the block will be transferred from one cache to the other via the high speed interconnect.

Simultaneous Writes on Different RAC Nodes

When one or more users need to modify the same block, this can lead to simultaneous writes being triggered on multiple nodes. These simultaneous writes lead to contention and require lock messaging and conversions. By using proper blocks sizes (large blocks aggravate the problem, small ones reduce it) this issue can be mitigated.  When using SSD with high bandwidth the duration of transactions, and thus the incidence of simultaneous writes are reduced.

Now that information has been presented on how SSD removes the bandwidth bottleneck for high access files, attention can now be turned to how to locate high concurrent access data files in an Oracle RAC database.

The above book excerpt is from:

Oracle RAC & Tuning with Solid State Disk

Expert Secrets for High Performance Clustered Grid Computing

ISBN 0-9761573-5-7

Donald K. Burleson & Mike Ault


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