Re: [PATCH 002 of 2] md: Improve the is_mddev_idle test

[Jan Engelhardt]

On May 10 2007 20:04, Neil Brown wrote:
>> >-		if ((curr_events - rdev->last_events + 4096) > 8192) {
>> >+		if ((long)curr_events - (long)rdev->last_events > 4096) {
>> > 			rdev->last_events = curr_events;
>> > 			idle = 0;
>> > 		}
>> 
>/* sync IO will cause sync_io to increase before the disk_stats
> * as sync_io is counted when a request starts, and 
> * disk_stats is counted when it completes.
> * So resync activity will cause curr_events to be smaller than
> * when there was no such activity.
> * non-sync IO will cause disk_stat to increase without
> * increasing sync_io so curr_events will (eventually)
> * be larger than it was before.  Once it becomes
> * substantially larger, the test below will cause
> * the array to appear non-idle, and resync will slow
> * down.
> * If there is a lot of outstanding resync activity when
> * we set last_event to curr_events, then all that activity
> * completing might cause the array to appear non-idle
> * and resync will be slowed down even though there might
> * not have been non-resync activity.  This will only
> * happen once though.  'last_events' will soon reflect
> * the state where there is little or no outstanding
> * resync requests, and further resync activity will
> * always make curr_events less than last_events.
> *
> */
>
>Does that read at all well?

It is a more verbose explanation of your patch description, yes.


	Jan
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