linux-kernel - Re: [PATCH] f2fs: merge WRITE bio into previous WRITE

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Date:   Fri, 2 Sep 2016 15:33:33 +0800
From:   Chao Yu <yuchao0@...wei.com>
To:     Jaegeuk Kim <jaegeuk@...nel.org>, <linux-kernel@...r.kernel.org>,
        <linux-fsdevel@...r.kernel.org>,
        <linux-f2fs-devel@...ts.sourceforge.net>
Subject: Re: [PATCH] f2fs: merge WRITE bio into previous WRITE_SYNC

Hi Jaegeuk,

On 2016/8/27 8:53, Jaegeuk Kim wrote:
> This can avoid bio splits due to different op_flags.

I thought about this, but I think this is not a good idea to increase merging
ratio of pages in bio. It breaks the rule of SYNC/ASYNC IO defined by system
which indicate degree of IO emergency, finally, some/more non-emergent IO will
treated as emergent one by IO scheduler, it will interrupt SYNC IOs in block
layer, more seriously, it may make real SYNC IO starvation.

Thanks,

> 
> Signed-off-by: Jaegeuk Kim <jaegeuk@...nel.org>
> ---
>  fs/f2fs/data.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
> index 7c8e219..c7c2022 100644
> --- a/fs/f2fs/data.c
> +++ b/fs/f2fs/data.c
> @@ -267,6 +267,11 @@ void f2fs_submit_page_mbio(struct f2fs_io_info *fio)
>  
>  	down_write(&io->io_rwsem);
>  
> +	/* WRITE can be merged into previous WRITE_SYNC */
> +	if (io->bio && io->last_block_in_bio == fio->new_blkaddr - 1 &&
> +			io->fio.op == fio->op && io->fio.op_flags == WRITE_SYNC)
> +		fio->op_flags = WRITE_SYNC;
> +
>  	if (io->bio && (io->last_block_in_bio != fio->new_blkaddr - 1 ||
>  	    (io->fio.op != fio->op || io->fio.op_flags != fio->op_flags)))
>  		__submit_merged_bio(io);
>