Re: Recover RAID6 with 4 disks removed

Am 06.02.20 um 14:46 schrieb Nicolas Karolak:

I have (had...) a RAID6 array with 8 disks and tried to remove 4 disks
from it, and obviously i messed up. Here is the commands i issued (i
do not have the output of them):

didn't you realize that RAID6 has redundancy to survive *exactly two*
failing disks no matter how many disks the array has anmd the data and
redundancy informations are spread ove the disks?

mdadm --manage /dev/md1 --fail /dev/sdh
mdadm --manage /dev/md1 --fail /dev/sdg
mdadm --detail /dev/md1
cat /proc/mdstat
mdadm --manage /dev/md1 --fail /dev/sdf
mdadm --manage /dev/md1 --fail /dev/sde
mdadm --detail /dev/md1
cat /proc/mdstat
mdadm --manage /dev/md1 --remove /dev/sdh
mdadm --manage /dev/md1 --remove /dev/sdg
mdadm --manage /dev/md1 --remove /dev/sde
mdadm --manage /dev/md1 --remove /dev/sdf
mdadm --detail /dev/md1
cat /proc/mdstat
mdadm --grow /dev/md1 --raid-devices=4
mdadm --grow /dev/md1 --array-size 7780316160  # from here it start
going wrong on the system

becaue mdadm din't't prevent you from shoot yourself in the foot, likely
for cases when one needs a hammer for restore from a uncommon state as
last ressort

set more than one disk at the same time to "fail" is aksing for troubles
no matter what

what happens when one drive starts to puke when you removed every
redundancy and happily start a reshape that implies heavy IO?

I began to have "inpout/output" error, `ls` or `cat` or almost every
command was not working (something like "/usr/sbin/ls not found").
`mdadm` command was still working, so i did that:

mdadm --manage /dev/md1 --re-add /dev/sde
mdadm --manage /dev/md1 --re-add /dev/sdf
mdadm --manage /dev/md1 --re-add /dev/sdg
mdadm --manage /dev/md1 --re-add /dev/sdh
mdadm --grow /dev/md1 --raid-devices=8

The disks were re-added, but as "spares". After that i powered down
the server and made backup of the disks with `dd`.

Is there any hope to retrieve the data? If yes, then how?

unlikely - the started reshape did writes