On Mon, 14 Dec 2009 11:56:32 -0500
Paul Clements [off-list ref] 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?

 Or possibly a counter reporting the number of requests that were not
 treated as 'write_behind' because the max had been reached.  Then you could
 increase the max_write_behind until the counter stopped increasing - or it's
 rate of increase dropped to an acceptable level.

 Does any of that work for you?

NeilBrown