[ruby-core:33075] [Ruby 1.9-Feature#4015] File::DIRECT Constant for O_DIRECT

Issue #4015 has been updated by Run Paint Run Run.

File io-advise.patch added

Last suggestion from me, I promise. ;-)

O_DIRECT is a controversial (<http://lkml.org/lkml/2007/1/11/121>) flag, and Linus and others recommend (<http://lkml.org/lkml/2007/1/10/233>) using madvise(2)/posix_fadvise(2) instead. The latter is, as the name suggests, part of the POSIX standard, avoiding the current problem of defining platform-specific, non-standard flags. Its advantages are summarised in O'Reilly's _Linux System Programming_:

<<A handful of common application workloads can readily benefit from a little well-
intentioned advice to the kernel. Such advice can go a long way toward mitigating
the burden of I/O. With hard disks being so slow, and modern processors being so
fast, every little bit helps, and good advice can go a long way.

Before reading a chunk of a file, a process can provide the POSIX_FADV_WILLNEED hint
to instruct the kernel to read the file into the page cache. The I/O will occur asyn-
chronously, in the background. When the application ultimately accesses the file, the
operation can complete without generating blocking I/O.

Conversely, after reading or writing a lot of data—say, while continuously streaming
video to disk—a process can provide the POSIX_FADV_DONTNEED hint to instruct the
kernel to evict the given chunk of the file from the page cache. A large streaming
operation can continually fill the page cache. If the application never intends to
access the data again, this means the page cache will be filled with superfluous data,
at the expense of potentially more useful data. Thus, it makes sense for a streaming
video application to periodically request that streamed data be evicted from the
cache.

A process that intends to read in an entire file can provide the POSIX_FADV_SEQUENTIAL
hint, instructing the kernel to perform aggressive readahead. Conversely, a process
that knows it is going to access a file randomly, seeking to and fro, can provide the
POSIX_FADV_RANDOM hint, instructing the kernel that readahead will be nothing but
worthless overhead.>>

Implementing posix_fadvise(2) would avoid complicating the logic of `open` any more, and at the same time provide a more general solution than O_DIRECT. The attached patch defines IO#advise as a wrapper around posix_fadvise(2). As this advice is never binding, and #respond_to? returns false for :advise on platforms that don't support it, it is trivial to write portable code that only invokes #advise where supported. Granted, this solution still requires defining constants. However, there is no danger of defining them on all platforms because platforms that don't support this syscall will raise a NotImplementedError for #advise.
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