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<title>Berkeley DB Reference Guide: Environment</title>
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<h3>Berkeley DB Reference Guide: Environment</h3>
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<h1 align=center>Shared Memory Regions</h1>
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Each of the Berkeley DB subsystems is described by one or more shared memory
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regions. These regions live in the environment home directory, and
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contain all of the shared information, including mutexes, that describes
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the Berkeley DB environment.
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The Berkeley DB library uses the POSIX mmap (or other similar) interface to map
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the shared memory regions. Most remote file systems (e.g., the Network
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File System (NFS) and the Andrew File System (AFS)), do not support
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mapping files into process memory. For this reason, we strongly recommend
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that the database home directory reside in a local filesystem.
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For remote file systems that do allow system files to be mapped into
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process memory, it is important to note that home directories accessed
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via remote file systems cannot be used simultaneously from multiple
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clients. None of the commercial remote file systems available today
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implement a coherent, distributed shared memory paradigm for
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remote-mounted files. As a result, different machines will see different
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versions of these shared regions and the system behavior is undefined.
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Databases, log files and temporary files may be placed on remote
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filesystems, although the application may incur a performance penalty
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