linux-kernel - Re: [PATCH 03/13] bcache: inherit the optimal I/O size

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Date:   Mon, 21 Sep 2020 16:00:10 +0200
From:   Christoph Hellwig <hch@....de>
To:     Coly Li <colyli@...e.de>
Cc:     Christoph Hellwig <hch@....de>, Jens Axboe <axboe@...nel.dk>,
        Song Liu <song@...nel.org>,
        Hans de Goede <hdegoede@...hat.com>,
        Richard Weinberger <richard@....at>,
        Minchan Kim <minchan@...nel.org>,
        Johannes Thumshirn <Johannes.Thumshirn@....com>,
        Justin Sanders <justin@...aid.com>,
        linux-mtd@...ts.infradead.org, dm-devel@...hat.com,
        linux-block@...r.kernel.org, linux-bcache@...r.kernel.org,
        linux-kernel@...r.kernel.org, drbd-dev@...ts.linbit.com,
        linux-raid@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, cgroups@...r.kernel.org
Subject: Re: [PATCH 03/13] bcache: inherit the optimal I/O size

On Mon, Sep 21, 2020 at 05:54:59PM +0800, Coly Li wrote:
> I am not sure whether virtual bcache device's optimal request size can
> be simply set like this.
> 
> Most of time inherit backing device's optimal request size is fine, but
> there are two exceptions,
> - Read request hits on cache device
> - User sets sequential_cuttoff as 0, all writing may go into cache
> device firstly.
> For the above two conditions, all I/Os goes into cache device, using
> optimal request size of backing device might be improper.
> 
> Just a guess, is it OK to set the optimal request size of the virtual
> bcache device as the least common multiple of cache device's and backing
> device's optimal request sizes ?

Well, if the optimal I/O size is wrong, the read ahead size also is
wrong.  Can we just drop the setting?