Re: problems with lots of arrays

[Mike Lovell <mike.lovell@endurance.com>]

On Fri, May 6, 2016 at 12:43 AM, NeilBrown <nfbrown@novell.com> wrote:
> I know why newer kernels don't seem to support more than 512 array.
>
> Commit: af5628f05db6 ("md: disable probing for md devices 512 and over.")
>
>
> You can easily use many more md devices by using a newish mdadm and
> setting
>
>    CREATE names=yes
>
> in /etc/mdadm.conf
>
> You cannot use names like "md512" because that gets confusing, but any
> name that isn't a string of digits is fine.  e.g. create /dev/md/foo
> and the array will be named "md_foo" in the kernel rather than "md127".
>
> I guess this qualifies as a regression and regressions are bad.....
> But I really wanted to be able to have arrays that didn't get magically
> created simply because you open a file in /dev.  That just leads to
> races with udev.
>
> The magic number "512" appears three times in the kernel.
>
>                 /* find an unused unit number */
>                 static int next_minor = 512;
>
> and
>
>         blk_register_region(MKDEV(MD_MAJOR, 0), 512, THIS_MODULE,
>                             md_probe, NULL, NULL);
> and
>         blk_unregister_region(MKDEV(MD_MAJOR,0), 512);
>
> A boot parameter which set that to something larger would probably be OK
> and would solve your immediate problem.
>
> But if you could transition to using named arrays instead of numbered
> arrays - even if that are "/dev/md/X%d", that would be be good I think.
>
> NeilBrown

we actually do specify the name to mdadm --create and mdadm --assemble
and have a naming scheme from our own internal tools. the problem we
were running into was that mdadm would auto-generate a minor number
that was invalid but we also don't have "CREATE names=yes" in
mdadm.conf. i'll have to experiment with that one.

thanks
mike