linux-kernel - Re: [PATCH] md/raid10: fix missing discard IO accounting

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Message-Id: <761DA302-0E21-4CDD-ACCE-85958DBD48D5@coly.li>
Date: Tue, 25 Mar 2025 19:48:24 +0800
From: Coly Li <i@...y.li>
To: Yu Kuai <yukuai1@...weicloud.com>
Cc: Coly Li <colyli@...nel.org>,
 song@...nel.org,
 jgq516@...il.com,
 linux-raid@...r.kernel.org,
 linux-kernel@...r.kernel.org,
 yi.zhang@...wei.com,
 yangerkun@...wei.com,
 "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH] md/raid10: fix missing discard IO accounting



> 2025年3月25日 15:27，Yu Kuai <yukuai1@...weicloud.com> 写道：
> 
> Hi,
> 
> 在 2025/03/25 15:04, Coly Li 写道:
>> On Tue, Mar 25, 2025 at 09:57:46AM +0800, Yu Kuai wrote:
>>> From: Yu Kuai <yukuai3@...wei.com>
>>> 
>>> md_account_bio() is not called from raid10_handle_discard(), now that we
>>> handle bitmap inside md_account_bio(), also fix missing
>>> bitmap_startwrite for discard.
>>> 
>>> Test whole disk discard for 20G raid10:
>>> 
>>> Before:
>>> Device   d/s     dMB/s   drqm/s  %drqm d_await dareq-sz
>>> md0    48.00     16.00     0.00   0.00    5.42   341.33
>>> 
>>> After:
>>> Device   d/s     dMB/s   drqm/s  %drqm d_await dareq-sz
>>> md0    68.00  20462.00     0.00   0.00    2.65 308133.65
>>> 
>>> Fixes: 528bc2cf2fcc ("md/raid10: enable io accounting")
>>> Signed-off-by: Yu Kuai <yukuai3@...wei.com>
>> Should we treat discard request as real I/O?
>> Normally IMHO discard request should not be counted as real data transfer,
>> correct me if I am wrong.
> 
> Normally it's not, that's why discard IOs are accounted separately in
> the block layer.
> 
> Also notice that discard should be treated as write, because after
> discard, reading will get zero data.

I see, it is for the separated discard counting.  Sure, I reply my Acked-by on the original patch.

Thank you for the explanation.

Coly Li