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

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Message-ID: <20210104084415.GA28741@lst.de>
Date:   Mon, 4 Jan 2021 09:44:15 +0100
From:   Christoph Hellwig <hch@....de>
To:     Ming Lei <ming.lei@...hat.com>
Cc:     linux-kernel@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, linux-block@...r.kernel.org,
        Christoph Hellwig <hch@....de>, Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH] fs/buffer: try to submit writeback bio in unit of page

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.