Re: MDP major registration

[Hubert Verstraete <hubskml@free.fr>]

Bill Davidsen wrote:
> Luca Berra wrote:
>> On Tue, Mar 25, 2008 at 05:57:06PM +0100, Hubert Verstraete wrote:
>>> Neil Brown wrote:
>>>> On Thursday March 13, hubskml@free.fr wrote:
>>>>> Neil,
>>>>>
>>>>> What is the status of the major for the partitionable arrays ?
>>>>
>>>> automatically determined at runtime.
>>>>
>>>>> I see that it is 254, which is in the experimental section, 
>>>>> according to the official Linux device list 
>>>>> (http://www.lanana.org/docs/device-list/).
>>>>> Will there be an official registration ?
>>>>
>>>> No. Is there any need?
>>>
>>> I got this question in mind when I saw that mkfs.xfs source code was 
>>> referring to the MD major to tune its parameters on an MD device, 
>>> while it ignores MDP devices.
>>> If there were reasons to register MD, wouldn't they apply to MDP too ?
>>
>> i don't think so:
>> bluca@percy ~ $ grep mdp /proc/devices
>> 253 mdp
> 
> Why is it important to have XFS tune its parameters for md and not for 
> mdp? I don't understand your conclusion here, is tuning not needed for 
> mdp, or so meaningless that it doesn't matter, or that XFS code reads 
> /proc/devices, or ??? I note that device-mapper also has a dynamic 
> major, what does XFS make of that?

It reads from /proc/devices.

> I don't know how much difference tuning makes, but if it's worth doing 
> at all, it should be done for mdp as well, I would think.

Same thought. I wrote the patch for mkfs.xfs but did not publish it for 
two reasons:
1) MD is registered but not MDP. Now I understand, it's not a problem, 
we just need to read /proc/devices as device-mapper does.
2) Tuning XFS for MDP can be achieved through the mkfs.xfs options. With 
a few lines in shell, my XFS on MDP now has the same performance as XFS 
on MD.

Hubert