EXT2 块大小和单一文件最大大小的关系。

EXT2格式 块大小为1024字节的话，单一文件最大容量是16GB，块大小为4096字节的话，单一文件最大容量为2TB，请问各位大侠这个是怎么计算的啊，我钻死胡同里了。晕晕晕~~~~~~~

如果你愿意，看看kernel源代码吧
位置: ~/fs/ext2/supper.c
看这个函数：
C/C++ code

/*
 * Maximal file size.  There is a direct, and {,double-,triple-}indirect
 * block limit, and also a limit of (2^32 - 1) 512-byte sectors in i_blocks.
 * We need to be 1 filesystem block less than the 2^32 sector limit.
 */
static loff_t ext2_max_size(int bits)
{
    loff_t res = EXT2_NDIR_BLOCKS;
    /* This constant is calculated to be the largest file size for a
     * dense, 4k-blocksize file such that the total number of
     * sectors in the file, including data and all indirect blocks,
     * does not exceed 2^32. */
    const loff_t upper_limit = 0x1ff7fffd000LL;

    res += 1LL << (bits-2);
    res += 1LL << (2*(bits-2));
    res += 1LL << (3*(bits-2));
    res <<= bits;
    if (res > upper_limit)
        res = upper_limit;
    return res;
}

调用上面函数的地方：
C/C++ code

static int ext2_fill_super(struct super_block *sb, void *data, int silent)
{
  ...
  
  sb->s_maxbytes = ext2_max_size(sb->s_blocksize_bits);
  ...
}

其中supper block中定义的s_maxbytes就是用作限制文件大小的！！

查获定义知道: EXT2_NDIR_BLOCKS = 12;

所以当blocksize = 1024的时候，sb->s_blocksize_bits　＝ 10 (2^1024);
这时候的sb->s_maxbytes = ext2_max_size (10); 计算出来，就约等于2^34 bytes(即16GB)！

当blocksize = 4096的时候，res = 12 (2^12 = 4096) (约等于);
这时候 sb->s_maxbytes = ext3_max_size (12); 计算出来，约定于 2^42 bytes(即4TB)，
也就是res = 4G(约等于);计算知道res > upper_limit。而upper_limit约等于2T。所以两者取小。res = 2T;
这样最大文件大小 sb->s_maxbytes = 2T;

bshawk,很好，找了很久，学习一下