> async       All I/O to the file system should be done asyn-
>             chronously.  In the event of a crash, it is
>             impossible for the system to verify the integrity of
>             data on a file system mounted with this option.  You
>             should only use this option if you have an applica-
>             tion-specific data recovery mechanism, or are willing
>             to recreate the file system from scratch.
> 
> could it cause unrecoverable damage to filesystem or could i only lost data
> not currently written to disk?

Unrecoverable filesystem damage.  The filesystem and fsck agree the
order of certain filesystem operations to ensure that fsck can always
fix a filesystem after a crash.  The async option defeats that
agreement.

Regards,

Giles