关于 Linux 直接 I/O 的问题

在看《 The Linux Programming Interface 》（ Linux/Unix 系统编程手册）第 13 章，第 6 小节，其中的示例代码:

/* direct_read.c

   Demonstrate the use of O_DIRECT to perform I/O bypassing the buffer cache
   ("direct I/O").

   Usage: direct_read file length [offset [alignment]]

   This program is Linux-specific.
*/
#define _GNU_SOURCE     /* Obtain O_DIRECT definition from <fcntl.h> */
#include <fcntl.h>
#include <malloc.h>
#include "tlpi_hdr.h"

int
main(int argc, char *argv[])
{
    int fd;
    ssize_t numRead;
    size_t length, alignment;
    off_t offset;
    char *buf;

    if (argc < 3 || strcmp(argv[1], "--help") == 0)
        usageErr("%s file length [offset [alignment]]\n", argv[0]);

    length = getLong(argv[2], GN_ANY_BASE, "length");
    offset = (argc > 3) ? getLong(argv[3], GN_ANY_BASE, "offset") : 0;
    alignment = (argc > 4) ? getLong(argv[4], GN_ANY_BASE, "alignment") : 4096;

    fd = open(argv[1], O_RDONLY | O_DIRECT);
    if (fd == -1)
        errExit("open");

    /* memalign() allocates a block of memory aligned on an address that
       is a multiple of its first argument. By specifying this argument as
       2 * 'alignment' and then adding 'alignment' to the returned pointer,
       we ensure that 'buf' is aligned on a non-power-of-two multiple of
       'alignment'. We do this to ensure that if, for example, we ask
       for a 256-byte aligned buffer, we don't accidentally get
       a buffer that is also aligned on a 512-byte boundary. */

    buf = memalign(alignment * 2, length + alignment);
    if (buf == NULL)
        errExit("memalign");

    buf += alignment;

    if (lseek(fd, offset, SEEK_SET) == -1)
        errExit("lseek");

    numRead = read(fd, buf, length);
    if (numRead == -1)
        errExit("read");
    printf("Read %ld bytes\n", (long) numRead);

    exit(EXIT_SUCCESS);
}

是想展示以下三点：

• 用于传递数据的缓冲区，其内存边界必须对其为块大小的整数倍。
• 数据传输的开始的点，亦即文件和设备的偏移量，必须是块大小的整数倍。
• 待传递数据的长度必须是块大小的整数倍。

中间大块的注释里有一句话：

we ensure that 'buf' is aligned on a non-power-of-two multiple of 'alignment'.

请问这个用意是什么呢？（因为我感觉这是不必要的，为什么非得让 buf 的开始地址对其为非 2 的幂呢？）