Re: [PATCH] block: Improve io_opt limit stacking

[Damien Le Moal <Damien.LeMoal@wdc.com>]

On 2020/05/14 15:58, Damien Le Moal wrote:
> When devices with different physical sector sizes are stacked, the
> largest value is used as the stacked device physical sector size. For
> the optimal IO size, the lowest common multiple (lcm) of the underlying
> devices is used for the stacked device. In this scenario, if only one of
> the underlying device reports an optimal IO size, that value is used as
> is for the stacked device but that value may not be a multiple of the
> stacked device physical sector size. In this case, blk_stack_limits()
> returns an error resulting in warnings being printed on device mapper
> startup (observed with dm-zoned dual drive setup combining a 512B
> sector SSD with a 4K sector HDD).
> 
> To fix this, rather than returning an error, the optimal IO size limit
> for the stacked device can be adjusted to the lowest common multiple
> (lcm) of the stacked physical sector size and optimal IO size, resulting
> in a value that is a multiple of the physical sector size while still
> being an optimal size for the underlying devices.
> 
> This patch is complementary to the patch "nvme: Fix io_opt limit
> setting" which prevents the nvme driver from reporting an optimal IO
> size equal to a namespace sector size for a device that does not report
> an optimal IO size.
> 
> Suggested-by: Keith Busch <kbusch@kernel.org>
> Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
> ---
>  block/blk-settings.c | 7 ++-----
>  1 file changed, 2 insertions(+), 5 deletions(-)
> 
> diff --git a/block/blk-settings.c b/block/blk-settings.c
> index 9a2c23cd9700..9a2b017ff681 100644
> --- a/block/blk-settings.c
> +++ b/block/blk-settings.c
> @@ -561,11 +561,8 @@ int blk_stack_limits(struct queue_limits *t, struct queue_limits *b,
>  	}
>  
>  	/* Optimal I/O a multiple of the physical block size? */
> -	if (t->io_opt & (t->physical_block_size - 1)) {
> -		t->io_opt = 0;
> -		t->misaligned = 1;
> -		ret = -1;
> -	}
> +	if (t->io_opt & (t->physical_block_size - 1))
> +		t->io_opt = lcm(t->io_opt, t->physical_block_size);
>  
>  	t->raid_partial_stripes_expensive =
>  		max(t->raid_partial_stripes_expensive,
> 

Jens,

Any comment on this patch ?
Note: the patch the patch "nvme: Fix io_opt limit setting" is already queued for
5.8.

-- 
Damien Le Moal
Western Digital Research