Re: mdadm: use static major/minor numbers.

[NeilBrown <neilb@suse.de>]

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On Tue, 20 Nov 2012 12:36:52 +0000 Benjamin ESTRABAUD <be@mpstor.com> wrote:

> Hi!
> 
> I recently updated mdadm-2.6.9 to mdadm-3.2.6 on a system (own busybox 
> based distro) that had its kernel version updated from 2.6.35 to 3.4.
> 
> Everything works well apart from a feature I used that I seem to be 
> unable to reactivate:
> 
> In the past, to ensure knowing the number of possible RAID devices that 
> could be created on the system, I used static major:minor mappings by 
> building the /dev/md/dXX and /dev/md/dXXpXX entries (with major 254, and 
> three minors for each device for 3 partitions), allowing me to be 
> certain that 64 RAID devices could be created at all times.
> 
> When creating an array, the /dev/md/dXX and dXXpXX devices "nodes" would 
> be used (not recreated) and the major/minor number these devices point 
> to would be used for the actual MD block device.
> 
> This was a very handy feature. With the latest mdadm, I simply cannot do 
> that, as creating a /dev/md/dXX will in fact remove that device file and 
> symlink it to /dev/md_dXX. I then created /dev/md_dXX devices prior to 
> creating an array, and got the following error message: "mdadm: 
> /dev/md_d3 exists but looks wrong, please fix".
> 
> Looking through the source I can see that mdadm basically verifies if 
> the file's major/minor matches the one that it had planned for the 
> device, which in this case doesn't.
> 
> Is there any way to work around that? Or in fact, I don't actually 
> *need* to use static major/minor numbers, but I need to know in advance 
> how many RAIDs I'll be able to create provided I'll always use 2 
> partitions for them and can provide the -amdp2 argument to mdadm to make 
> sure that only 2 partitions devices nodes are created.
> 
> Is there a way to know the maximum number of RAIDs that can be created, 
> provided that nothing else uses major 9 and 254?
> 
> Thank you very much in advance for your help!

I don't really understand what your problem is.

mdadm should create any devices it needs - unless it detects udev, in which
case it leaves the device creation to udev.

So simply don't create any md devices in /dev and let mdadm do whatever is
required.
Does that approach not suit your needs?  If not, why not?

NeilBrown

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