Re: [PATCH v3] dm-cache watermarks

[Mike Snitzer <snitzer@redhat.com>]

On Sun, Mar 06 2016 at  4:39am -0500,
Steven Wilton <swilton@fluentit.com.au> wrote:

> I realised that I over-thought the original logic.  The attached
> patch is much simpler - write back dirty cache entries at the
> migration threshold rate when we are over the low watermark, and we
> write back as fast as possible when we go over the high watermark.
> 
> The current behaviour can be replicated by setting the high
> watermark to 100, and the low watermark to 0.
> 
> regards
> 
> Steven

...

> diff --git a/drivers/md/dm-cache-target.c b/drivers/md/dm-cache-target.c
> index 7dab682..7b8b4a3 100644
> --- a/drivers/md/dm-cache-target.c
> +++ b/drivers/md/dm-cache-target.c
> @@ -2332,6 +2364,8 @@ static void set_cache_size(struct cache *cache, dm_cblock_t size)
>  }
>  
>  #define DEFAULT_MIGRATION_THRESHOLD 2048
> +#define DEFAULT_WRITEBACK_HIGH_WATERMARK 40
> +#define DEFAULT_WRITEBACK_LOW_WATERMARK 30
>  

These should default to current behavior (100 and 0 respectively, like
you mentioned above).

This change needs an accompanying Documentation update.  It also needs a
proper path header to accurately convey the scope of the change.

Please post v4 with these changes and then I'll take a closer look (as
I'm sure Joe will).  But please be aware that in general we want less
knobs, not more.

Thanks,
Mike