linux-kernel - Re: [f2fs-dev] [PATCH 1/2] f2fs: set 4KB discard granularity by default

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Date:   Tue, 4 Sep 2018 23:36:14 +0900
From:   Ju Hyung Park <qkrwngud825@...il.com>
To:     Chao Yu <yuchao0@...wei.com>
Cc:     Jaegeuk Kim <jaegeuk@...nel.org>, linux-kernel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net
Subject: Re: [f2fs-dev] [PATCH 1/2] f2fs: set 4KB discard granularity by default

Hi,

I was wondering what would be the negatives on reducing the discard granularity
other than making discard more aggressive(hence higher overhead and latency?).

Thanks.

On Tue, Aug 14, 2018 at 4:08 PM Chao Yu <yuchao0@...wei.com> wrote:
>
> On 2018/8/14 12:13, Jaegeuk Kim wrote:
> > On 08/10, Chao Yu wrote:
> >> Small granularity (size < 64K) fragmentation will cause f2fs suspending
> >> all pending discards, result in performance regression, so let's set
> >> 4KB discard granularity by default.
> >>
> >> So that without fstrim, we also have the ability to avoid any performance
> >> regression caused by non-alignment mapping between fs and flash device.
> >
> > This is why we added a sysfs entry. Why do we need to change the default
> > value every time?
>
> Of course, I didn't forget it. But, mostly, our user didn't know very well about
> our filesystem including each configuration's meaning, mechanism, or usage
> scenario, most of the time, they will choose to test f2fs with all default
> option, and then make the conclusion.
>
> Currently, with default 64k granularity, if we simulate fragmentation scenario
> of filesystem, like by a)writing 4k file and b)deleting even indexing file, then
> all 4k discards won't be issued, result in exhaustion of free space of flash
> storage, and performance degradation.
>
> So I think we'd better to consider and set default value of configuration more
> elaborately to avoid obvious weakness.
>
> Thoughts?
>
> Thanks,
>
> >
> >>
> >> Signed-off-by: Chao Yu <yuchao0@...wei.com>
> >> ---
> >>  fs/f2fs/f2fs.h | 2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
> >> index 58431b9bfd8f..273ffdaf4891 100644
> >> --- a/fs/f2fs/f2fs.h
> >> +++ b/fs/f2fs/f2fs.h
> >> @@ -248,7 +248,7 @@ struct discard_entry {
> >>  };
> >>
> >>  /* default discard granularity of inner discard thread, unit: block count */
> >> -#define DEFAULT_DISCARD_GRANULARITY         16
> >> +#define DEFAULT_DISCARD_GRANULARITY         1
> >>
> >>  /* max discard pend list number */
> >>  #define MAX_PLIST_NUM               512
> >> --
> >> 2.18.0.rc1
> >
> > .
> >
>
>
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