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Database Essentials
Administrative Planning : Database areas : Data area optimization : Keeping extents sized to eliminate I/O indirection
Keeping extents sized to eliminate I/O indirection
In an unjournaled file system, I/O indirection happens when a single inode table cannot address all of the addresses within a file. The inode is a mapping of logical to physical addresses in a file. Think of the inode table as a table of contents for the file. In theory, a file can be large (more than 4GB) before indirection occurs, but the conditions need to be perfect. Inode indirection is not a concern for journaled file systems.
In the real world, it is possible to see indirection at a file size of 500MB, but on most systems you do not see indirection until 1GB or higher. The penalty for smaller database area extents is fairly low. Generally, you only need to verify that the number of file descriptors (open files) available from the operating system is high enough to support the extents.