为什么我创建的ram只能＜＝128M？

ctpyy_11

执行mke2fs  -m0 /dev/ram 450M -F 报错提示如下：
mke2fs 1.39 (29-May-2006)
Filesystem label=
OS type: Linux
Block size=4096 (log=2)
Fragment size=4096 (log=2)
115200 inodes, 115200 blocks
0 blocks (0.00%) reserved for the super user
First data block=0
Maximum filesystem blocks=121634816
4 block groups
32768 blocks per group, 32768 fragments per group
28800 inodes per group
Superblock backups stored on blocks:
        32768, 98304

mke2fs: 无效的参数 while zeroing block 115184 at end of filesystem
Writing inode tables: 1/4
Could not write 8 blocks in inode table starting at 32800: 无效的参数
但mke2fs  -m0 /dev/ram  128M -F 则可创建成功
为什么我创建的ram只能＜＝128M？

ctpyy_11

centos 5.1