Ankur Bose [off-list ref] writes:

Hi, it seems like this is very active group , I don't see any reason why anyone is not replying to the below mail.
 

Kindly reply we are in middle of something.

We are all in the middle of something.

Thanks,
ankur.
-----Original Message-----
From: Suresh Babu Kandukuru 
Sent: 12 October 2015 17:34
To: linux-raid@vger.kernel.org; neilb@suse.de
Subject: Scalability of MD raid 1 mirror devices


Dear Group,

We are doing scalability of MD raid 1  mirror devices on the Linux host running the 3.17.2 .   we see the number of RAID 1 devices limited to 128 in one case and 511 in another case .  we would like to know why this limitation ?. Appreciate any kind of inputs and pointers 

We can create the RAID 1 device in 3 ways.

1.  /dev/mdX   -> here X is the number, we can specify from 0 to 511.
2. /dev/md/X  -> here also X is a number from 0 to 511. It creates it as a link to the actual device /dev/mdX. ( similar to above but creates a link also).
3. /dev/md/”name”  -> This creates a link to actual device, whichever is free starting from 127 to 0. Below is the function which is responsible for it.

char *find_free_devnm(int use_partitions) {
        static char devnm[32];
        int devnum;
        for (devnum = 127; devnum != 128;  devnum = devnum ? devnum-1 : (1<<20)-1) {

                if (use_partitions)
                        sprintf(devnm, "md_d%d", devnum);
                else
                        sprintf(devnm, "md%d", devnum);
                if (mddev_busy(devnm))
                        continue;
                if (!conf_name_is_free(devnm))
                        continue;
                if (!use_udev()) {
                        /* make sure it is new to /dev too, at least as a
                         * non-standard */
                        int devid = devnm2devid(devnm);
                        if (devid) {
                                char *dn = map_dev(major(devid),
                                                   minor(devid), 0);
                                if (dn && ! is_standard(dn, NULL))
                                        continue;
                        }
                }
                break;
        }
        if (devnum == 128)
                return NULL;
        return devnm;
}

So ideally we should not create a device which is more than 128 { The
program may crash }.

Please explain why you think the program would crash?

Then we  tried to find how we are able to create up to 511 and why it is failing after that.

int dev_open(char *dev, int flags)

inside this function 

fd = open(dev, flags);  / this line is assigning fd to -1 , which is causing the program to fail. So I wrote a simple program to crosscheck it.

int main(){

        char devname[32] = "/dev/hello1";

// The flags I have set according to the code.

        int flags = O_RDWR;
        flags |= O_DIRECT;
        if (mknod(devname, S_IFBLK|0600, makedev(9,511)) == 0) {

                int  fd = open(devname, flags);
                cout<<fd<<endl;
                unlink(devname);

        }
}

So if the minor number is more than 511, the “fd” is assigned to -1,
if it is in the range of 0 to 511 It is working fine.

Hmmm... you are right.
Probably due to this:

	blk_register_region(MKDEV(MD_MAJOR, 0), 512, THIS_MODULE,
			    md_probe, NULL, NULL);

Try changing the "512" to "1<<MINORBITS".


NeilBrown