(PECL eio >= 0.0.1dev)
eio_readahead — Perform file readahead into page cache
$fd,$offset,$length,$pri = EIO_PRI_DEFAULT,$callback = NULL,$data = NULL
eio_readahead() populates the page cache with data from a file so that subsequent reads from
that file will not block on disk I/O. See READAHEAD(2) man page for details.
fdStream, Socket resource, or numeric file descriptor
offsetStarting point from which data is to be read.
lengthNumber of bytes to be read.
pri请求的优先级：EIO_PRI_DEFAULT，EIO_PRI_MIN，EIO_PRI_MAX 或 null。如果是
null，pri 将设为
EIO_PRI_DEFAULT。
callback
callback
函数在请求完成时被调用。其应匹配一下原型：
void callback(mixed $data, int $result[, resource $req]);data传递给请求的用户数据。
result针对请求的结果的值。通常是相应的系统调用返回的值。
req可选的请求资源，可被 eio_get_last_error() 之类的函数使用。
data
Arbitrary variable passed to callback.
eio_readahead() returns request resource on success, 或者在失败时返回 false.