Re: problems with lots of arrays

On Fri, May 6, 2016 at 12:43 AM, NeilBrown [off-list ref] 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