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To:     Christoph Hellwig <hch@lst.de>
Cc:     Jens Axboe <axboe@kernel.dk>, Song Liu <song@kernel.org>,
        Hans de Goede <hdegoede@redhat.com>,
        Richard Weinberger <richard@nod.at>,
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From:   Coly Li <colyli@suse.de>
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Subject: Re: [PATCH 03/13] bcache: inherit the optimal I/O size
Message-ID: <5bcc52dc-ca8f-bbdd-69ef-4b6312e7994a@suse.de>
Date:   Mon, 21 Sep 2020 23:09:48 +0800
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On 2020/9/21 22:00, Christoph Hellwig wrote:
> 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?
> 

I feel this is something should be fixed. Indeed I overlooked it until
you point out the issue now.

The optimal request size and read ahead pages hint are necessary, but
current initialization is simple. A better way might be dynamically
setting them depends on the cache mode and some special configuration.

By your inspiration, I want to ACK your original patch although it
doesn't work fine for all condition. Then we may know these two settings
(ra_pages and queue_io_opt) should be improved for more situations. At
lease for most part of the situations they provide proper hints.

How do you think of the above idea ?

Coly Li