why online shrink is not supported in EXT4_IOC_RESIZE_FS ioctl?

[joshua2009]

Hi,

I referred EXT4_IOC_RESIZE_FS ioctl implementation in linux kernel ver 3.3.8 source.

Online shrinking is not supported and the code is as below.

   if (n_blocks_count < o_blocks_count) {
      /* On-line shrinking not supported */
      ext4_warning(sb, "can't shrink FS - resize aborted");
      return -EINVAL;
   }

Can anyone please tell the reason why online shrinking is not supported.

Thanks in adavance.

Regards,
Raphel