Pro 6 won't boot up after going from 6.7.1 to 6.7.3

sure:

First of all:

DISCLAIMER: Commands given here are for tutorial reasons only - i give neither an implied nor expressive warranty that these commands will work as intended.

MORE OVER A WARNING: They might destroy the integrity of your data and you are obliged to make a backup before!

At first determine of which drives your md0 device is made of:

# mdadm --detail /dev/md0

/dev/md0:

        Version : 1.2

  Creation Time : Mon Apr 18 22:58:10 2016

     Raid Level : raid1

     Array Size : 4190208 (4.00 GiB 4.29 GB)

  Used Dev Size : 4190208 (4.00 GiB 4.29 GB)

   Raid Devices : 6

  Total Devices : 6

    Persistence : Superblock is persistent

    Update Time : Thu May 25 18:00:01 2017

          State : clean 

 Active Devices : 6

Working Devices : 6

 Failed Devices : 0

  Spare Devices : 0

(...)

           Name : 540ed549:0  (local to host 540ed549)

           UUID : 3201e354:b9fcd222:f4d2fe4d:888dc6f6

         Events : 19025

    Number   Major   Minor   RaidDevice State

      11       8        1        0      active sync   /dev/sda1

      10       8       17        1      active sync   /dev/sdb1

       5       8       33        2      active sync   /dev/sdc1

       4       8       49        3      active sync   /dev/sdd1

       3       8       81        4      active sync   /dev/sde1

       6       8       97        5      active sync   /dev/sdf1

assure that the drive is clean and no extensive i/o is on this drive. Warning: If the drives are in another enumaration (e.g. sda,  sdb, sdc, sdd, sdf, sdg) you need to adapt the commands to the changed drive letters.

Now mark the 4 drives you don't need yet as "failed"

# mdadm /dev/md0 --manage --fail /dev/sdf1

# mdadm /dev/md0 --manage --fail /dev/sde1

# mdadm /dev/md0 --manage --fail /dev/sdd1

# mdadm /dev/md0 --manage --fail /dev/sdc1

You can now savely tell the raid manager that those 4 drives aren't necessary any more and not part of this array anymore:

# mdadm --grow --backup-file=/data/md0backup/tmpfile --raid-devices=2 /dev/md0

Now we remove and add 3 drives to mark them as "spares" and remove the last drive only, because we migrate from RAID1 to RAID5 and afterwards to RAID6.

# mdadm /dev/md0 --manage --remove /dev/sdc1 --add /dev/sdc1

# mdadm /dev/md0 --manage --remove /dev/sdd1 --add /dev/sdd1

# mdadm /dev/md0 --manage --remove /dev/sde1 --add /dev/sde1

# mdadm /dev/md0 --manage --remove /dev/sdg1 

now we are ready to rumble: Please give a backup file for this operation in case anything goes wrong during the very first part of the migration:

We are now migrating a two disk RAID1 into a 2 disk RAID5 (yes it works this way with mdadm!)

# mdadm --grow --backup-file=/data/md0backup/tmpfile --level=5 /dev/md0

and wait until the raid system resynched and shows "clean" :

# mdadm --detail /dev/md0

/dev/md0:

        Version : 1.2

  Creation Time : Mon Apr 18 22:58:10 2016

     Raid Level : raid5

     Array Size : 4190208 (4.00 GiB 4.29 GB)

  Used Dev Size : 4190208 (4.00 GiB 4.29 GB)

   Raid Devices : 2

  Total Devices : 5

    Persistence : Superblock is persistent

    Update Time : Thu May 25 18:00:01 2017

          State : clean 

 Active Devices : 2

Working Devices : 2

 Failed Devices : 0

  Spare Devices : 0

         Layout : left-symmetric

     Chunk Size : 64K

           Name : 540ed549:0  (local to host 540ed549)

           UUID : 3201e354:b9fcd222:f4d2fe4d:888dc6f6

         Events : 19025

    Number   Major   Minor   RaidDevice State

      11       8        1        0      active sync   /dev/sda1

      10       8       17        1      active sync   /dev/sdb1

You'll get a warning that some data  had to be stored into the given backup file!

Now we extend our RAID5 with the 3 additional spares:

# mdadm --grow --backup-file=/data/md0backup/tmpfile --raid-devices=5 /dev/md0

please wait until the synchronization succeeds:

# mdadm --detail /dev/md0

/dev/md0:

        Version : 1.2

  Creation Time : Mon Apr 18 22:58:10 2016

     Raid Level : raid5

     Array Size : 16760832 (15.98 GiB 17.16 GB)

  Used Dev Size : 4190208 (4.00 GiB 4.29 GB)

   Raid Devices : 5

  Total Devices : 5

    Persistence : Superblock is persistent

    Update Time : Thu May 25 18:00:01 2017

          State : clean 

 Active Devices : 5

Working Devices : 5

 Failed Devices : 0

  Spare Devices : 0

         Layout : left-symmetric

     Chunk Size : 64K

           Name : 540ed549:0  (local to host 540ed549)

           UUID : 3201e354:b9fcd222:f4d2fe4d:888dc6f6

         Events : 19025

    Number   Major   Minor   RaidDevice State

      11       8        1        0      active sync   /dev/sda1

      10       8       17        1      active sync   /dev/sdb1

       5       8       33        2      active sync   /dev/sdc1

       4       8       49        3      active sync   /dev/sdd1

       3       8       81        4      active sync   /dev/sde1

Now we add the last spare to it and make it RAID6:

# mdadm /dev/md0 --manage --add /dev/sdf1

# mdadm --grow --backup-file=/data/md0backup/tmpfile --level=6 /dev/md0

Side note: the raid grow to level 6 will automatically adjust the raid-devices parameter to 6.

Again please wait until the raid is synchronized and the new layout ist cleanly available:

# mdadm --detail /dev/md0

/dev/md0:

        Version : 1.2

  Creation Time : Mon Apr 18 22:58:10 2016

     Raid Level : raid6

     Array Size : 16760832 (15.98 GiB 17.16 GB)

  Used Dev Size : 4190208 (4.00 GiB 4.29 GB)

   Raid Devices : 6

  Total Devices : 6

    Persistence : Superblock is persistent

    Update Time : Thu May 25 18:00:01 2017

          State : clean 

 Active Devices : 6

Working Devices : 6

 Failed Devices : 0

  Spare Devices : 0

         Layout : left-symmetric

     Chunk Size : 64K

           Name : 540ed549:0  (local to host 540ed549)

           UUID : 3201e354:b9fcd222:f4d2fe4d:888dc6f6

         Events : 19025

    Number   Major   Minor   RaidDevice State

      11       8        1        0      active sync   /dev/sda1

      10       8       17        1      active sync   /dev/sdb1

       5       8       33        2      active sync   /dev/sdc1

       4       8       49        3      active sync   /dev/sdd1

       3       8       81        4      active sync   /dev/sde1

       6       8       97        5      active sync   /dev/sdf1

The last step is to tell the filesystem it has now a different size:

# btrfs filesystem resize max /

check if it is applied:

# df -h

Filesystem      Size  Used Avail Use% Mounted on

udev             10M  4.0K   10M   1% /dev

/dev/md0         16G  1.2G   15G   8% /

tmpfs           3.9G     0  3.9G   0% /dev/shm

tmpfs           3.9G  2.6M  3.9G   1% /run

tmpfs           2.0G  1.1M  2.0G   1% /run/lock

tmpfs           3.9G     0  3.9G   0% /sys/fs/cgroup

/dev/md127       19T  5.2T   14T  28% /data

/dev/md127       19T  5.2T   14T  28% /home

/dev/md127       19T  5.2T   14T  28% /apps

You are now set!