Finally, the solution:
as mentioned earlier, I reverted the reshape
mdadm --assemble /dev/md0 --force --verbose --update=revert-reshape
--invalid-backup /dev/sda1 /dev/sdc1 /dev/sdd1 /dev/sde1 /dev/sdf1
This restarted the md auto-read-only. So I had to manually start the md
mdadm --readwrite /dev/md0
which resulted in bad superblock.
mke2fs won´t work here, because it´s limited to 16TB of diskspace, where
my md is 36TiB big. Because I´m a bit lazy to google some more, I
started gparted at the machine, choosed the md0 und initiated filesystem
integrity check.
This took 5 minutes, afterwards I was able to mount my md.
So I unmounted the md again in order to grow it again:
root@nas:~# mdadm --grow --raid-devices=5 /dev/md0
--backup-file=/tmp/bu_neu.bak
mdadm: Need to backup 6144K of critical section..
root@nas:~# mdadm -D /dev/md0
/dev/md0:
Version : 1.2
Creation Time : Sun May 17 00:23:42 2020
Raid Level : raid5
Array Size : 35156256768 (33527.62 GiB 36000.01 GB)
Used Dev Size : 11718752256 (11175.87 GiB 12000.00 GB)
Raid Devices : 5
Total Devices : 5
Persistence : Superblock is persistent
Intent Bitmap : Internal
Update Time : Tue May 26 02:08:22 2020
State : clean, reshaping
Active Devices : 5
Working Devices : 5
Failed Devices : 0
Spare Devices : 0
Layout : left-symmetric
Chunk Size : 512K
Consistency Policy : bitmap
Reshape Status : 0% complete
Delta Devices : 1, (4->5)
Name : nas:0 (local to host nas)
UUID : d7d800b3:d203ff93:9cc2149a:804a1b97
Events : 38631
Number Major Minor RaidDevice State
0 8 1 0 active sync /dev/sda1
1 8 33 1 active sync /dev/sdc1
2 8 49 2 active sync /dev/sdd1
4 8 65 3 active sync /dev/sde1
5 8 81 4 active sync /dev/sdf1
root@nas:~#
As we see in mdadm details, reshaping is running. Have a look at mdstat:
root@nas:~# cat /proc/mdstat
Personalities : [raid6] [raid5] [raid4] [linear] [multipath] [raid0]
[raid1] [raid10]
md0 : active raid5 sda1[0] sdf1[5] sde1[4] sdd1[2] sdc1[1]
35156256768 blocks super 1.2 level 5, 512k chunk, algorithm 2
[5/5] [UUUUU]
[>....................] reshape = 0.8% (93989376/11718752256)
finish=8260.4min speed=23454K/sec
bitmap: 0/88 pages [0KB], 65536KB chunk
unused devices: <none>
Seems to be a bit slowly right now. I had expected a speed arount
60MByte due to 6G-Sata drives. However.. it´s running again.
I will dig into speed later.
Again: Thanks to everyone who helped me with ideas and / or advice. You
guys saved my ass :)
