使用statvfs结构计算可用空间的最佳方法？

Question

我们是否必须使用

1. f_bfree×f_frsize    或
2. f_bfree×f_bsize

计算文件系统中的可用磁盘空间

struct statvfs {
    unsigned long  f_bsize;    /* file system block size */
    unsigned long  f_frsize;   /* fragment size */
    fsblkcnt_t     f_blocks;   /* size of fs in f_frsize units */
    fsblkcnt_t     f_bfree;    /* # free blocks */
    fsblkcnt_t     f_bavail;   /* # free blocks for unprivileged users */
    fsfilcnt_t     f_files;    /* # inodes */
    fsfilcnt_t     f_ffree;    /* # free inodes */
    fsfilcnt_t     f_favail;   /* # free inodes for unprivileged users */
    unsigned long  f_fsid;     /* file system ID */
    unsigned long  f_flag;     /* mount flags */
    unsigned long  f_namemax;  /* maximum filename length */
};