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Extract data from iSCSI LUN or return back my /volume1

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some time ago on my xpenology 6.1.5 i got RAID1 crashed due to power failure.
then I sure did the most stupidiest thing - didn't made a backup of hdd before trying to recover raid...
and after week of trying different things i have following:
1) /VOLUME2 - single 2tb HDD used for non-vital stuff - working okay. i even backed up almost all important data from RAID1 - it was in 'readonly' state for a while :)
2) /VOLUME1 - crashed
two RAID1 hdds. disk A and disk B.
- disk A is almost okay, but says system volume crashed,
- disk B - after trying to reassemble raid to make it think it contains one drive only - i ended up with 'recreating' raid and clearing superblock.
(I wanted to reassemble with mdadm -assemble -disks=1, but instead made --create, so it cleared superblock :( )

now synology doesnt see volume1 at all. and shows raid is crashed.

Now the need - i have 3 iSCSI LUNs
- one non growing - and i just took the image file from @iSCSI folder and successfully copied all the data from inside
- two more - looks like they are in strange ADVANCED format and growing-style.

I cannot understand how to mount them or copy (ls shows unreal file sizes like 100 GB). I tried to check with 'btrfs --show' but it doesnt recognize any filesystem inside.
I have really important data there, how can i copy or mount those stange LUNs?

p.s. i had no spare drive to add to rebuild raid classically by adding new disk - and also was worried about stressing existing drives - both are 45.000 hrs+ online.

Edited by AntonV
adding info about hdd age
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I restored my /volume1 myself! Huray!
Okay, so, after week of investigations I tried different tools to extract data. And here whats happened.

1) The only tool which states it can extract data from Synology LUNs is Reclaime and ReclamePro(which is 4 times more expensive).
However, I tried and it failed. I was talking to their support and one of their senior developers started investigating, why its not working (and doing it still, they are very interested in this case).

2) What is funny, i actually could restore it a week ago if kept cold blooded :) (Actually no, i didn't know one command line option)
So, previously I already disassembled my 2-disk RAID1 with
# Stop the RAID1
mdadm -S /dev/md2
# Recreate a RAID1 with just one disk keeping the data as is
mdadm --create --assume-clean --level=1 --force --raid-devices=1 /dev/md2 /dev/sdd5
At this point i stopped last time, since LVM couldnt find volume groups and volumes...
Now to the happy finish:
#Stop the RAID1 again
mdadm --stop /dev/md2
# Recreate RAID1 with new option "--metadata=1.2"
mdadm --create --level=1 --force --raid-devices=1 /dev/md2 /dev/sdd5 --metadata=1.2
# Reload LVM configuration from backup, where vg1000 is the name of my volume group
vgcfgrestore --test -f /etc/lvm/backup/vg1000 vg1000
# that was dry run, to check that its okay, now real run
vgcfgrestore -f /etc/lvm/backup/vg1000 vg1000
#Output: Restored volume group vg1000
# check it is in the list now
vgs -v
# finally make it active.
vgchange -ay vg1000
# after this my /volume1 is restored as RAID1 mirror with only one HDD.
# but DSM is not picking it up properly - reboot is needed.
After reboot all work fine. Storage manager shows my raid, however is has 'Failed system partition'. Its an easy fix for me.
Looking at the disks with lsblk its seen /dev/md0 (system raid1 ) doesnt include partition from my hdd ( /dev/sdd1 ).
mdadm --manage /dev/md0 --add /dev/sdd1
And all is okay after few minutes.

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if you don't want to mess around with lvm then use raid1 or raid5/6 directly and don't use SHR1/2 (or synologys automatic that usually creates shr's)

(i planned from the start to not mess around with asymmetric disks size and shr, so i created a clean raid5 from the start and extended to raid6 alter that with more disks)

if you plan to recover manually or to use another nas system as fallback a "keep it simple and compatible" approach is to prefer (shr is not that kind of unique, it should work with other systems too as lvm is also part of every linux but keeping it KISS makes it easier to handle if things go wrong - like messed up dsm install by update, just boot OMV from a prepared disk or usb and get access to your data instantly)

Edited by IG-88
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