linux-kernel - Re: [PATCH] fs/buffer: try to submit writeback bio in unit of page

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Date:   Mon, 4 Jan 2021 17:24:25 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     Christoph Hellwig <hch@....de>
Cc:     linux-kernel@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, linux-block@...r.kernel.org,
        Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH] fs/buffer: try to submit writeback bio in unit of page

On Mon, Jan 04, 2021 at 09:44:15AM +0100, Christoph Hellwig wrote:
> On Wed, Dec 30, 2020 at 08:08:15AM +0800, Ming Lei wrote:
> > It is observed that __block_write_full_page() always submit bio with size of block size,
> > which is often 512 bytes.
> > 
> > In case of sequential IO, or >=4k BS random/seq writeback IO, most of times IO
> > represented by all buffer_head in each page can be done in single bio. It is actually
> > done in single request IO by block layer's plug merge too.
> > 
> > So check if IO represented by buffer_head can be merged to single page
> > IO, if yes, just submit single bio instead of submitting one bio for each buffer_head.
> 
> There is some very weird formatting in here.  From a very quick look
> the changes look sensible, but I wonder if we should spend so much
> time optimizing the legacy buffer_head I/O path, rather than switching
> callers to saner helpers.

It may take long to convert fs code into iomap, and I understand fs/block_dev.c
can't be converted to iomap until all FS removes buffer_head, correct me
if it is wrong.


Thanks,
Ming