mdadm issue adding components to an array (avail_size / array_size issue).

[Benjamin ESTRABAUD <be@mpstor.com>]

Hi,

I am experiencing what seems to be a bug with mdadm which prevents me 
from --add ing a disk in some specifics conditions.

The current setup is as follow:

-1 RAID 5 on 3* 26Gb block devices. > /dev/md/d0
-1 RAID 5 on 3*36Gb block devices. > /dev/md/d1
-1 RAID 5 on 3* 9Gb block devices. > /dev/md/d2

No config file is being used. RAIDs are created as follow:

mdadm - v2.6.9 - 10th March 2009

./mdadm --create -vvv --force --run --metadata=1.2 /dev/md/dX --level=5 
--size=<sizeofraid> --chunk=64 --name=<name, like: 1356341> -n3 
--bitmap=internal --bitmap-chunk=4096 --layout=ls /dev/<blockdev1> 
/dev/<blockdev2> /dev/<blockdev3>

- Several different size available block devices for adding to the 
arrays (1*14Gb, 1*26Gb, 2*32Gb, etc.)

If trying to --add a block device to the /dev/md/d0 RAID array after 
degrading it, everything works fine as long as the device being added is 
at least as big as the "component_size" size found in sysfs from 
/dev/md_d0/md/component_size. Therefore, a 32Gb drive can be added to 
the first array.

However, trying to do the same procedure for the third RAID, using 
either a 9Gb, 14Gb block device fails complaining that the device being 
hot added is not large enough to join the array. Which is strange, since 
after checking the /dev/md_d3/md/component_size, this value is much 
lower than the size obtained for the block device being added.

On another hand, degrading md/d1 and trying to add a 32Gb block device 
to this array composed of 3*36Gb block devices does not complain that 
the block device size is not large enough to join the array, and adds it 
to /dev/md/d1, however, as a Failed (F) drive.

In the second example, the hotAdd does not work on /dev/md/d1 that has 
its smallest component size set to 9Gb as long as the drive being added 
is not >= to the component size of /dev/md/d0's component size.

After further checking in the mdadm source, I noticed that "array_size" 
in Manage_subdevs from Manage.c is always the same, regardless of which 
RAID we are trying to operate on.

by examining the "get_component_size" method, I noticed the following:

   if (major(stb.st_rdev) != get_mdp_major())
        sprintf(fname, "/sys/block/md%d/md/component_size",
            (int)minor(stb.st_rdev));
    else
        sprintf(fname, "/sys/block/md_d%d/md/component_size",
            (int)minor(stb.st_rdev)>>MdpMinorShift);"

 >>> (int)minor(stb.st_rdev)>>MdpMinorShift) is always "0", therefore 
the component size file is always the following:

/sys/block/md_d0/md/component_size

Whatever the md device is currently used, md/d1, or md/d2 etc.

The "get_component_size" seems to be using an integer, "fd" to find out 
the size and return it.
However, fd is always the same value, "3", whatever RAID is being worked on.

this value seems to be generated in mdadm.c, in the main function:

line 944: mdfd = open_mddev(devlist->devname, autof);

This always returns "3" in my case.

I was wondering what exactly this "mdfd" corresponded to, and if the 
fact that it never changes is normal or not. I am wondering whether the 
issue lies with this variable, or if it does in the get_component_size 
function.

Would anyone have experienced a similar issue here?

Thank you very much in advance for your comments/advices.

Ben.