Fwd: max_queued_requests for raid1 and raid10 - questions

Hello,
Sorry for noise, missclick.

Mariusz


-------- Forwarded Message --------
Subject: max_queued_requests for raid1 and raid10 - questions
Date: Tue, 29 Jun 2021 10:37:58 +0200
From: Tkaczyk, Mariusz <redacted>
To: NeilBrown <redacted>

Hello Neil,
I have some questions related to max_queued_requests implemented by you
for raid1 and raid10. See code below:

/* When there are this many requests queue to be written by
  * the raid thread, we become 'congested' to provide back-pressure
  * for writeback.
  */
static int max_queued_requests = 1024;


It was added years ago:
https://git.kernel.org/pub/scm/linux/kernel/git/song/md.git/commit
/?id=34db0cd60f8a1f4ab73d118a8be3797c20388223

I've reached out scenario with cache in write-only mode where
this limiter degrades performance significantly (around 4 times).
I used Open-CAS:
https://github.com/Open-CAS/open-cas-linux

So, at this point I have some basic questions:
Is "back-pressure" still a case? Do you know any scenario where it
brings benefits?
If yes, I'll move this parameter to sysfs, to make it configurable
via mdadm config file (using SYSFS line) per array.
What do you think?

 From other hand, shall be consider to bump this value up? It seems to
be small today.

TIA,
Mariusz