linux-kernel - Re: [PATCH] md/raid0: add config parameters to specify zone layout

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Message-ID: <E3616A45-C6D0-4B3B-8112-688B03126F00@fb.com>
Date:   Thu, 30 Apr 2020 06:19:26 +0000
From:   Song Liu <songliubraving@...com>
To:     Jason Baron <jbaron@...mai.com>
CC:     Coly Li <colyli@...e.de>, "agk@...hat.com" <agk@...hat.com>,
        "snitzer@...hat.com" <snitzer@...hat.com>,
        "linux-raid@...r.kernel.org" <linux-raid@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Guoqing Jiang <guoqing.jiang@...ud.ionos.com>,
        NeilBrown <neilb@...e.de>
Subject: Re: [PATCH] md/raid0: add config parameters to specify zone layout

Hi Jason,

> On Apr 27, 2020, at 2:10 PM, Jason Baron <jbaron@...mai.com> wrote:
> 
> 
> 
> On 4/25/20 12:31 AM, Coly Li wrote:
>> On 2020/3/26 23:28, Jason Baron wrote:
>>> Let's add some CONFIG_* options to directly configure the raid0 layout
>>> if you know in advance how your raid0 array was created. This can be
>>> simpler than having to manage module or kernel command-line parameters.
>>> 
>> 
>> Hi Jason,
>> 
>> If the people who compiling the kernel is not the end users, the
>> communication gap has potential risk to make users to use a different
>> layout for existing raid0 array after a kernel upgrade.
>> 
>> If this patch goes into upstream, it is very probably such risky
>> situation may happen.
>> 
>> The purpose of adding default_layout is to let *end user* to be aware of
>> they layout when they use difference sizes component disks to assemble
>> the raid0 array, and make decision which layout algorithm should be
>> used. Such situation cannot be decided in kernel compiling time.
> 
> I agree that in general it may not be known at compile time. Thus,
> I've left the default as RAID0_LAYOUT_NONE. However, there are
> use-cases where it is known at compile-time which layout is needed.
> In our use-case, we knew that we didn't have any pre-3.14 raid0
> arrays. Thus, we can safely set RAID0_ALT_MULTIZONE_LAYOUT. So
> this is a simpler configuration for us than setting module or command
> line parameters.

I would echo Coly's concern that CONFIG_ option could make it risky. 
If the overhead of maintaining extra command line parameter, I would
recommend you carry a private patch for this change. For upstream, it
is better NOT to carry the default in CONFIG_.

Thanks,
Song