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ORACLE IO ARCHITECTURE generate, create ean 13 none in .net projects iPhone We ve encountered man EAN 13 for .NET y varieties of Oracle IO in previous chapters, as we ve sought to find ways to avoid various types of IO. Now that we are attacking IO directly, we need to have a clear and precise understanding of the various forms Oracle IO can take.

Figure 21-1 shows the major types of Oracle IO, the processes that participate, and the associated wait events. DATAFILE SINGLE BLOCK READ A single block read occurs when Oracle needs to retrieve a single block of information from a data file. This operation is recorded by the wait interface as db file sequential read.

2. 2 This is an incredib ly badly named wait because from the disk perspective, what occurs is not a sequential read but a random read.. Disk IO Tuning Fundamentals Direct Path Read/Writ e Temp Oracle Server Process Database File Sequential/Scattered Read Database Writer (DBWR) Recovery Writer (RVWR). Direct Path Read/Write Database File Parallel Write Database Files Database Files Flashback Log File Write Temporary Tablespace Files Flashback Logs Active Redo Log Inactive Redo Logs Archived Logs RMAN Backup and Recovery I/O Backup Destination Log File Parallel Wri .NET EAN-13 Supplement 5 te Log File Sequential Read Log Writer (LGWR) Archiver (ARCH) Log Archive I/O RMAN RMAN Backup and Recovery I/O. FIGURE 21-1. Overview of Oracle IO and IO waits. Single block reads ar e most obviously associated with index lookups; Oracle performs a series of single block reads while navigating the index structure (see 5, Indexing and Clustering, ) that eventually supplies the address of the required table blocks on disk that are then also retrieved by single block reads. MULTI BLOCK READ In a multi-block read, Oracle retrieves multiple blocks of data in a single operation. This is recorded by the wait interface as db file scattered read.

Despite the inference provided by the term scattered, multiblock reads involve a set of contiguous blocks and are used when Oracle scans consecutive blocks typically during a full table scan or an index scan.. 21 The maximum numbe EAN13 for .NET r of blocks that can be read in a single operation is defined by the parameter DB_FILE_MULTIBLOCK_READ_COUNT. The default value for this parameter is operating system- and block size-specific and is usually set to a value that aligns with the maximum operating system IO size, typically 512K or 1M.

DIRECT PATH READS Db file sequential reads and db file scattered read operations are both buffer cached reads. The blocks are read-only if they are not found in the buffer cache and once read are added to the buffer cache. We discussed the operation of the buffer cache in some depth in 18, Buffer Cache Tuning.

Direct path read operations do not involve the buffer cache. The Oracle server process instead reads blocks directly from the data files into server process private memory (the PGA). Direct path reads avoid some of the overhead and contention involved with buffer cache management but, unlike in a buffer cache read, the data read is unavailable to other processes.

If another process happens to need the same blocks, they will need to be reread from disk. We also discussed direct path operations in 18. Oracle uses direct path reads in the following circumstances: Always when performing temporary segment IO.

Usually when performing parallel query. In Oracle 10g, Oracle uses direct path reads by default for all parallel operations. In Oracle 11g, Oracle can use buffer cache IO during parallel execution if the optimizer calculates an advantage.

In 11g, Oracle sometimes uses direct path reads during serial queries when it calculates an advantage in doing so. We discussed the circumstances in which this might occur in 17, Shared Memory Contention. .

Direct path reads can be single block or multiblock. However, direct path reads are more often utilized when scan operations are performed: Oracle favors buffered IO when performing indexed single-block reads because the probability of the block being reused in the near future is high. Consequently direct path reads are most often multiblock in nature.

TEMPORARY DIRECT PATH IO We discussed the nature of temporary segment IO in several earlier chapters: in 11, Sorting, Grouping, and Set Operations, while optimizing sorting and in 19, Optimizing PGA Memory, when optimizing PGA memory. When a sort, hash join, or other operation requires memory workspace and insufficient PGA memory is available, data must be written to and read from temporary segments. Oracle uses direct path IO for this purpose to avoid creating buffer cache contention.

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