Re: MDP major registration

[Bill Davidsen <davidsen@tmr.com>]

Hubert Verstraete wrote:
> 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.

Hopefully the following patch will be picked up by vendors. Now from the 
linux-kernel list there have been two recent changes in similar areas, 
the ability of loop to support partitionable devices, and the ability to 
have nbd handle partitionable devices. Are XFS users so lucky that this 
change would also improve things for those media?

-- 
Bill Davidsen <davidsen@tmr.com>
  "Woe unto the statesman who makes war without a reason that will still
  be valid when the war is over..." Otto von Bismark