On 12/5/20 11:52, Giuseppe Bilotta wrote:

On Mon, May 11, 2020 at 11:16 PM Guoqing Jiang
[off-list ref] wrote:

quoted

On 5/11/20 11:12 PM, Guoqing Jiang wrote:

quoted

On 5/11/20 10:53 PM, Giuseppe Bilotta wrote:

quoted

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.

But basically, the array can't process any new requests when checking is

Yeah, locking the entire device might be excessive (especially if it's
a big one). Using a granularity larger than 1 but smaller than the
whole device could be a compromise. Since the “no lock” approach seems
to be about an order of magnitude faster (at least in Piergiorgio's
benchmark), my guess was that something between 8 and 16 could bring
the speed up to be close to the “no lock” case without having dramatic
effects on I/O. Reading all 8/16 stripes before processing (assuming
sufficient memory) might even lead to better disk utilization during
the check.

I know very little about this, but could you perhaps lock 2 x 16 
stripes, and then after you complete the first 16, release the first 16, 
lock the 3rd 16 stripes, and while waiting for the lock continue to 
process the 2nd set of 16?

Would that allow you to do more processing and less waiting for 
lock/release?

Regards,
Adam