Re: [Ext2-devel] Re: [PATCH 2/3] ext3 reservation allow turn off for specifed file

[Andrew Morton <akpm@osdl.org>]

Mingming Cao <cmm@us.ibm.com> wrote:
>
> 	/*
> +	 * if user passed a seeky-access hint to kernel,
> +	 * through POSIX_FADV_RANDOM,(file->r_ra.ra_pages is cleared)
> +	 * turn off reservation for block allocation correspondingly.
> +	 *
> +	 * Otherwise, if user switch back to POSIX_FADV_SEQUENTIAL or
> +	 * POSIX_FADV_NORMAL, re-open the reservation window.
> +	 */
> +	windowsz = atomic_read(&EXT3_I(inode)->i_rsv_window.rsv_goal_size);
> +	if ((windowsz > 0) && (!file->f_ra.ra_pages))
> +		atomic_set(&EXT3_I(inode)->i_rsv_window.rsv_goal_size, -1);
> +	if ((windowsz == -1) && file->f_ra.ra_pages)
> +		atomic_set(&EXT3_I(inode)->i_rsv_window.rsv_goal_size,
> +					EXT3_DEFAULT_RESERVE_BLOCKS);
> +

It's pretty sad that we add this extra code into ext3_prepare_write() -
it's almost never actually executed.

I wonder how important this optimisation really is?  I bet no applications
are using posix_fadvise(POSIX_FADV_RANDOM) anyway.

It we really see that benefits are available from this approach we probably
need to bite the bullet and add file_operations.fadvise()