why online shrink is not supported in EXT4_IOC_RESIZE_FS ioctl?

Hi,

I referred EXT4_IOC_RESIZE_FS ioctl implementation in linux kernel ver
3.3.8.

Implementation specifies tht Online shrinking is not supported. 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
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