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

[John Stoffel]

>>>>> "Song" == Song Liu <songliubraving@xxxxxx> writes:

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