On Fri, 15 May 2015 03:00:24 -0400 (EDT) Xiao Ni [off-list ref] wrote:

Hi Neil

   I encounter the problem when I reshape a 4-disks raid5 to raid5. It just can
appear with loop devices.

   The steps are:

[root@dhcp-12-158 mdadm-3.3.2]# mdadm -CR /dev/md0 -l5 -n5 /dev/loop[0-4] --assume-clean
mdadm: /dev/loop0 appears to be part of a raid array:
       level=raid5 devices=6 ctime=Fri May 15 13:47:17 2015
mdadm: /dev/loop1 appears to be part of a raid array:
       level=raid5 devices=6 ctime=Fri May 15 13:47:17 2015
mdadm: /dev/loop2 appears to be part of a raid array:
       level=raid5 devices=6 ctime=Fri May 15 13:47:17 2015
mdadm: /dev/loop3 appears to be part of a raid array:
       level=raid5 devices=6 ctime=Fri May 15 13:47:17 2015
mdadm: /dev/loop4 appears to be part of a raid array:
       level=raid5 devices=6 ctime=Fri May 15 13:47:17 2015
mdadm: Defaulting to version 1.2 metadata
mdadm: array /dev/md0 started.
[root@dhcp-12-158 mdadm-3.3.2]# mdadm /dev/md0 -a /dev/loop5
mdadm: added /dev/loop5
[root@dhcp-12-158 mdadm-3.3.2]# mdadm --grow /dev/md0 --raid-devices 6
mdadm: Need to backup 10240K of critical section..
[root@dhcp-12-158 mdadm-3.3.2]# cat /proc/mdstat 
Personalities : [raid6] [raid5] [raid4] 
md0 : active raid5 loop5[5] loop4[4] loop3[3] loop2[2] loop1[1] loop0[0]
      8187904 blocks super 1.2 level 5, 512k chunk, algorithm 2 [6/6] [UUUUUU]
      [>....................]  reshape =  0.0% (0/2046976) finish=6396.8min speed=0K/sec
      
unused devices: <none>

   It because the sync_max is set to 0 when run the command --grow

[root@dhcp-12-158 mdadm-3.3.2]# cd /sys/block/md0/md/
[root@dhcp-12-158 md]# cat sync_max 
0

   I tried reproduce with normal sata devices. The progress of reshape is no problem. Then
I checked the Grow.c. If I use sata devices, in function reshape_array, the return value
of set_new_data_offset is 0. But if I used loop devices, it return 1. Then it call the function
start_reshape. 

set_new_data_offset returns '0' if there is room on the devices to reduce the
data offset so that the reshape starts writing to unused space on the array.
This removes the need for a backup file, or the use of a spare device to
store a temporary backup.
It returns '1' if there was no room for relocating the data_offset.

So on your sata devices (which are presumably larger than your loop devices)
there was room.  On your loop devices there was not.

   In the function start_reshape it set the sync_max to reshape_progress. But in sysfs_read it
doesn't read reshape_progress. So it's 0 and the sync_max is set to 0. Why it need to set the
sync_max at this? I'm not sure about this. 

sync_max is set to 0 so that the reshape does not start until the backup has
been taken.
Once the backup is taken, child_monitor() should set sync_max to "max".

Can you  check if that is happening?

Thanks,
NeilBrown

   I tried to fix this but I'm not sure whether it's the right way. I'll send the patches in 
other mails.

Best Regards
Xiao
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