linux-kernel - Re: [PATCH] blk-throttle: support io merge over iops

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Message-ID: <Z88K5JtR4rhhIFsY@slm.duckdns.org>
Date: Mon, 10 Mar 2025 05:53:08 -1000
From: Tejun Heo <tj@...nel.org>
To: Ming Lei <ming.lei@...hat.com>
Cc: Yu Kuai <yukuai1@...weicloud.com>, axboe@...nel.dk,
	josef@...icpanda.com, linux-block@...r.kernel.org,
	linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
	yi.zhang@...wei.com, yangerkun@...wei.com,
	"yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH] blk-throttle: support io merge over iops_limit

Hello,

On Mon, Mar 10, 2025 at 10:16:46AM +0800, Ming Lei wrote:
...
> > Yes, but it's a litter hard to explain to users the differece between IO
> > split and IO merge, they just think IO split is the numer of IOs issued
> > to disk, and IO merge is the number of IOs issued from user.
> 
> Here it is really one trouble.
> 
> a) Sometimes people think IOs wrt. IOPS means that the read/write IO
> submitted from application, one typical example is `fio`.
> 
> b) Sometimes people think it is the data observed from `iostat`.
> 
> In a), io merge/split isn't taken into account, but b) does cover io
> merge and split.
> 
> So question is that what is the correct way for user to observe IOPS
> wrt. iops throttle?

If we could go back in time, I think the right metric to use is
hardware-seen IOPS as that's better correlated with the actual capacity
available (the correlation is very flawed but still better) and the number
of issued bio's can easily change depending on kernel implementation
details.

That said, I'm not sure about changing the behavior now. blk-throtl has
mostly used the number of bios as long as it has existed and given that
there can be a signficant difference between the two metrics, I'm not sure
the change is justified at this point.

Thanks.

-- 
tejun