On Mon, 14 Dec 2009 11:56:32 -0500 Paul Clements <paul.clements@xxxxxxxxxxxx> 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 -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
