Re: raid6check extremely slow ?

[Guoqing Jiang <guoqing.jiang@cloud.ionos.com>]

On 5/11/20 10:53 PM, Giuseppe Bilotta wrote:
> Hello Piergiorgio,
>
> On Mon, May 11, 2020 at 6:15 PM Piergiorgio Sartor
> <piergiorgio.sartor@nexgo.de> wrote:
>> Hi again!
>>
>> I made a quick test.
>> I disabled the lock / unlock in raid6check.
>>
>> With lock / unlock, I get around 1.2MB/sec
>> per device component, with ~13% CPU load.
>> Wihtout lock / unlock, I get around 15.5MB/sec
>> per device component, with ~30% CPU load.
>>
>> So, it seems the lock / unlock mechanism is
>> quite expensive.
>>
>> I'm not sure what's the best solution, since
>> we still need to avoid race conditions.
>>
>> Any suggestion is welcome!
> Would it be possible/effective to lock multiple stripes at once? Lock,
> say, 8 or 16 stripes, process them, unlock. I'm not familiar with the
> internals, but if locking is O(1) on the number of stripes (at least
> if they are consecutive), this would help reduce (potentially by a
> factor of 8 or 16) the costs of the locks/unlocks at the expense of
> longer locks and their influence on external I/O.
>

Hmm, maybe something like.

check_stripes
	
	-> mddev_suspend
	
	while (whole_stripe_num--) {
		check each stripe
	}
	
	-> mddev_resume


Then just need to call suspend/resume once.

Thanks,
Guoqing