Scalability of MD raid 1 mirror devices

[Suresh Babu Kandukuru <suresh.babu.kandukuru@oracle.com>]

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 }.

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.

So should we go with the range of 511 or stick to 128 ?

Thanks 
/Suresh
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