Re: Unable to (un)-grow raid6

[Phil Turmel <philip@turmel.org>]

On 12/05/2015 10:04 PM, David Waite wrote:
> I’m having difficulty shrinking down a RAID6 array (md2) on a Sinology NAS. I wish to go from 13 drives to 11, and believe I need to go to 12 first to maintain operation and redundancy through the resizing process.

No, you can go straight to 11 if you've set array-size properly.  --grow
operations maintain redundancy throughout.

> The -array-size has already been shrunk to account for the two drives I wish to remove, and one of the devices has been removed and the machine restarted before the command output below was generated.

You might need to set array-size again.  It is a temporary setting that
only becomes permanent when --grow successfully starts.

> # cat /proc/mdstat 
> Personalities : [linear] [raid0] [raid1] [raid10] [raid6] [raid5] [raid4] 
> md2 : active raid6 sda3[0] sdia3[14] sdib3[9] sdic3[10] sdid3[11] sdie3[12] sdg3[17] sdf3[5] sde3[16] sdd3[3] sdc3[13] sdb3[15]
>       26329895935 blocks super 1.2 level 6, 64k chunk, algorithm 2 [13/12] [UUUUUUU_UUUUU]
>       
> md1 : active raid1 sda2[0] sdb2[1] sdc2[2] sdd2[3] sde2[4] sdf2[5] sdg2[6]
>       2097088 blocks [8/7] [UUUUUUU_]
>       
> md0 : active raid1 sda1[0] sdb1[1] sdc1[2] sdd1[3] sde1[4] sdf1[5] sdg1[6]
>       2490176 blocks [8/7] [UUUUUUU_]
>       
> unused devices: <none>


> When running 
> # mdadm --grow -n 12 /dev/md2 --backup-file=/mnt/backup-file3
> mdadm: max_devs [384] of [/dev/md2]
> mdadm: Need to backup 7040K of critical section..
> mdadm: Cannot set device shape for /dev/md2: Invalid argument
> 
> ---
> 
> dmesg just reports:
> [87908.606245] md: couldn't update array info. -22

Error code 22 is "Invalid Argument", corresponding to the mdadm output.

Try setting array size again, then try again.  Feel free to go straight
to 11 devices -- it'll save you much time.

Also, for completeness, show --detail and --examine for the array and it
members.

Phil

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