----- Original Message -----

From: "Xiao Ni" <redacted>
To: linux-raid@vger.kernel.org
Cc: "jes sorensen" <redacted>
Sent: Wednesday, September 3, 2014 1:59:08 PM
Subject: Will the write request starve?

Hi all

   I meet a problem and I can't find the answer. Could you help me?
When raid5d want to handle a request, it need to get stripes by
__get_priority_stripe.
In the __get_priority_stripe function, it always try to get stripe from
handle_list.

   Make an assumption that the raid of level 5 is assembled with 4 disks, and
   the raid
is normal. Now the a write request just want to write a single chunk, so we
choose RMW

Sorry, it'll choose RCW

and the stripe will put on the delay_list, then activate it and put it to
hold_list.
But if there are many read requests after the write request, when raid5d
handle stripes
it will get stripe from handle_list. So will the write request starve?

   Certainly I know it will not, because many tests have been done. But I
   can't find
the reason why it will not starve.

   And there is a variable bypass_count, what's the function about it? If
   bypass_count <
bypass_threshold, it can get a chance to wait for the next write request to
get a full write.
So avoid read requests. Am I right?

Best Regards
Xiao
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