readahead - initiate file readahead into page cache
readahead() initiates readahead on a file so that subsequent reads from that file will be satisfied from the cache, and not block on disk I/O (assuming the readahead was initiated early enough and that other activity on the system did not in the meantime flush pages from the cache).
fd argument is a file descriptor identifying the file
which is to be read. The
offset argument specifies the starting
point from which data is to be read and
count specifies the
number of bytes to be read. I/O is performed in whole pages, so that
offset is effectively rounded down to a page boundary and bytes
are read up to the next page boundary greater than or equal to
(offset+count). readahead() does not read
beyond the end of the file. The file offset of the open file description
referred to by the file descriptor
fd is left unchanged.
On success, readahead() returns 0; on failure, -1 is
errno set to indicate the cause of the
fd is not a valid file descriptor or is not open for
fd does not refer to a file type to which
readahead() can be applied.
The readahead() system call appeared in Linux 2.4.13; glibc support has been provided since version 2.3.
The readahead() system call is Linux-specific, and its use should be avoided in portable applications.
On some 32-bit architectures, the calling signature for this system call differs, for the reasons described in syscall(2).
readahead() attempts to schedule the reads in the background and return immediately. However, it may block while it reads the filesystem metadata needed to locate the requested blocks. This occurs frequently with ext on large files using indirect blocks instead of extents, giving the appearance that the call blocks until the requested data has been read.
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