linux-kernel - Re: [dm-devel] [PATCH 3/4] dm-qcow2: Introduce driver to create block devices over QCOW2 files

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Message-ID: <241b972d-5f78-6282-4f67-131a2566d479@openvz.org>
Date:   Tue, 29 Mar 2022 18:24:17 +0300
From:   Kirill Tkhai <kirill.tkhai@...nvz.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     agk@...hat.com, snitzer@...hat.com, dm-devel@...hat.com,
        song@...nel.org, linux-kernel@...r.kernel.org,
        khorenko@...tuozzo.com, axboe@...nel.dk,
        linux-block@...r.kernel.org
Subject: Re: [dm-devel] [PATCH 3/4] dm-qcow2: Introduce driver to create block
 devices over QCOW2 files

On 29.03.2022 16:34, Christoph Hellwig wrote:
> On Mon, Mar 28, 2022 at 02:18:35PM +0300, Kirill Tkhai wrote:
>> The driver is request based, since this allows to use blk-mq
>> merging of request. Driver splits requests itself, and every
>> request (i.e., qio) after splitting fits a single cluster.
>> (In some cases it is worth to create bigger splits, and this
>> is a subject of further optimizations).
> 
> Nak, please don't do that.  If you want finer grained processing use
> a bio based driver, not a request based one.  This is just getting us
> into tons of problems.

Could you explain what you mean? Why shouldn't I use generic bio merging
code, but implementing my own merging? Which problems you point?

Generic blk-mq is well tested and lots of people work on its performance.
It's not obvious reason I should better implement own realization.