Re: [PATCH 2/3] md/raid5-cache: bump flush stripe batch size

NeilBrown
2017-03-03T03:03:31Z

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On Fri, Feb 17 2017, Shaohua Li wrote:

> Bump the flush stripe batch size to 2048. For my 12 disks raid
> array, the stripes takes:
> 12 * 4k * 2048 = 96MB
>
> This is still quite small. A hardware raid card generally has 1GB size,
> which we suggest the raid5-cache has similar cache size.
>
> The advantage of a big batch size is we can dispatch a lot of IO in the
> same time, then we can do some scheduling to make better IO pattern.
>
> Last patch prioritizes stripes, so we don't worry about a big flush
> stripe batch will starve normal stripes.
>
> Signed-off-by: Shaohua Li <shli@fb.com>
> ---
> drivers/md/raid5-cache.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/md/raid5-cache.c b/drivers/md/raid5-cache.c
> index 3f307be..b25512c 100644
> --- a/drivers/md/raid5-cache.c
> +++ b/drivers/md/raid5-cache.c
> @@ -43,7 +43,7 @@
> /* wake up reclaim thread periodically */
> #define R5C_RECLAIM_WAKEUP_INTERVAL (30 * HZ)
> /* start flush with these full stripes */
> -#define R5C_FULL_STRIPE_FLUSH_BATCH 256
> +#define R5C_FULL_STRIPE_FLUSH_BATCH 2048

Fixed numbers are warning signs... I wonder if there is something better
we could do? "conf->max_nr_stripes / 4" maybe? We use that sort of
number elsewhere.
Would that make sense?

Thanks,
NeilBrown

> /* reclaim stripes in groups */
> #define R5C_RECLAIM_STRIPE_GROUP (NR_STRIPE_HASH_LOCKS * 2)
>
> --
> 2.9.3

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