[Bug 241385] databases/sqlite3: enable OS features

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=241385

Thomas Munro <tmunro at FreeBSD.org> changed:

           What    |Removed                     |Added
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                 CC|                            |tmunro at FreeBSD.org

--- Comment #12 from Thomas Munro <tmunro at FreeBSD.org> ---
"The operating system must have a unified buffer cache in order for the
memory-mapped I/O extension to work correctly, especially in situations where
two processes are accessing the same database file and one process is using
memory-mapped I/O while the other is not. Not all operating systems have a
unified buffer cache. In some operating systems that claim to have a unified
buffer cache, the implementation is buggy and can lead to corrupt databases."

Note that FreeBSD with ZFS doesn't have a unified buffer cache (mmap'd files
are duplicated in the ARC and regular kernel buffers).  I don't know whether
it's safe or not or how the performance compares to pwrite/pread, for sqlite3. 
Also, I noticed that other distributions like Debian are not enabling the mmap
option, and are going with pread/pwrite despite their unified buffer cache.

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