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Date:   Thu, 30 Apr 2020 12:09:49 -0400
From:   "John Stoffel" <john@stoffel.org>
To:     Song Liu <songliubraving@fb.com>
Cc:     Jason Baron <jbaron@akamai.com>, Coly Li <colyli@suse.de>,
        "agk\@redhat.com" <agk@redhat.com>,
        "snitzer\@redhat.com" <snitzer@redhat.com>,
        "linux-raid\@vger.kernel.org" <linux-raid@vger.kernel.org>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Guoqing Jiang <guoqing.jiang@cloud.ionos.com>,
        NeilBrown <neilb@suse.de>
Subject: Re: [PATCH] md/raid0: add config parameters to specify zone layout
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>>>>> "Song" == Song Liu <songliubraving@fb.com> writes:

Song> Hi Jason,
>> On Apr 27, 2020, at 2:10 PM, Jason Baron <jbaron@akamai.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.

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

I agree as well.  Just because you have a known base, doesn't mean
that others wouldn't be hit with this problem.

John