Neil,

Thanks for the feedback...

Neil Brown wrote:
> On Mon, 14 Dec 2009 11:56:32 -0500
> Paul Clements <paul.clements@steeleye.com> wrote:
> 
>> This is a very simple patch that exposes the behind_writes counter of an 
>> md array via a sysfs entry. This is helpful when tuning the value of 
>> behind_writes. Knowing, for instance, the maximum value over time allows 
>> one to set a proper upper value for an array.
>>
> 
> Hi Paul,
>  while I can see that this information could be useful, I wonder if this is
>  the best way of accessing it.
>  I imagine that the count of behind_writes would change quite quickly, so you
>  would need to sample it at quite a high rate to get a meaningful number.
> 
>  I'm very conscious that the 'active_stripes' number provided by RAID5 is
>  close to useful as it almost always reads a 'zero' even when the array is
>  quite busy.
> 
>  Maybe having a sysfs attribute that reports the maximum value, and allow
>  that maximum to be reset by writing to the attribute?

OK, I did this. I called it behind_writes_top. I also left the 
behind_writes entry, since it might be useful too.

See attached.

Thanks,
Paul