linux-kernel - Re: [f2fs-dev] [PATCH] f2fs: fix double count on issued discard commands

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Date:   Wed, 13 Sep 2017 15:13:06 +0800
From:   Chao Yu <yuchao0@...wei.com>
To:     <daeho.jeong@...sung.com>, Jaegeuk Kim <jaegeuk@...nel.org>
CC:     "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-f2fs-devel@...ts.sourceforge.net" 
        <linux-f2fs-devel@...ts.sourceforge.net>
Subject: Re: [f2fs-dev] [PATCH] f2fs: fix double count on issued discard
 commands

On 2017/9/12 12:34, Daeho Jeong wrote:
>> Yeah, that's exactly like what I made a mistake before.
>> I should have mentioned that earlier. :)
> 
> Or I think the previous code which used "iter++" might be right.
> You might just want to check the fixed number of small discards, DISCARD_ISSUE_RATE,
> when issue_cond is "true".
> 
> Anyways, I have another question about this function.
> How about just issuing, not checking whether it is idle, the fixed number of small
> discards, DISCARD_ISSUE_RATE, when issue_cond is "true".
> Actually, the discard commands will be issued as "asynchronous" requests,
> which has a low priority in the I/O scheduler,
> so the performance degradation of other threads by doing this will not be much severe,
> but we can make the performance of the storage device better even if there is no idle.

Actually, we didn't change priority of discard command, so that it is still
synchronous IO for I/O scheduler, hence I/O interference will still exist if we
try to issue discard without IO aware ability.

Of course we can change the priority of discard command to lower, but potential
issue is that with ROW I/O scheduler in kernel or FTL, async I/O will handle
very slowly in heavy load scenario, if we are going to trigger sync write IO in
place in where we're doing async discard, we will face long latency.

Still I think it is worth to build the ability to issue async discard as a part
of discard policy and later we can adjust policy based on different scenario.

Thanks,

> 
> I am just worried about the storage device I/O performance gets worse 
> under I/O intensive senario where there is no idle
> 
> Thanks,
> 
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