Hi everyone,I'm trying to recover data from a Nextcloud storage volume after an unexpected failure and I'm running out of ideas. The goal is data recovery only (mostly family photos). I don't care about restoring the container itself.
I found a few older threads with similar symptoms, but my case seems different because the thin metadata and the ext4 filesystem both appear to be intact.
Environment
- Proxmox VE 8.2.4
- Kernel: 6.8.8-2-pve
- LVM Thin Pool
- One physical disk (8 TB)
- LXC container with an additional mount:
The thin LV:
/dev/nextcloud/vm-102-disk-0<br>
is no longer activatable.
Error
lvchange -ay nextcloud/vm-102-disk-0<br><br>device-mapper: reload ioctl failed: No data available<br>Kernel log:
device-mapper: table: thin: Couldn't open thin internal device (-ENODATA)<br>
Current LVM status
LV VG Attr<br>nextcloud nextcloud twi---tz--<br>vm-102-disk-0 nextcloud Vwi---tz--<br>The pool itself exists, but the thin volume cannot be activated.
What I already verified
1. thin_check
thin_check /dev/mapper/nextcloud-nextcloud_tmeta<br>Completes successfully:
examining superblock<br>examining devices tree<br>examining mapping tree<br>checking space map counts<br>
No errors.
2. thin_dump
The metadata contains the thin device:<device dev_id="1" mapped_blocks="62120451"><br>
So Device ID 1 still exists.
3. thin_rmap
thin_rmap --region 16387..16388 metadata.bin<br>returns
data 16387..16388 -> thin(1) 0..1<br>
The mappings appear valid.
4. ext4 filesystem
I manually read the ext4 superblock from the mapped data blocks.dumpe2fs reports:
- valid ext4 filesystem
- filesystem state: clean
- journal present
- valid UUID
- valid group descriptors
- last mounted: 2026-06-14
- last write: 2026-06-15
I also manually verified the Group Descriptor Table via dd and the values look correct.
What does NOT work
lvchange -ay nextcloud/vm-102-disk-0<br>vgchange -ay nextcloud<br>
DMDE only detects the LVM physical volume but cannot automatically expose the ext4 filesystem because the thin device itself cannot be instantiated.
What I am trying to achieve
I do not need the container.I only want read-only access to the ext4 filesystem so I can copy the photos to another disk.
My question
Has anyone seen a case where- thin_check succeeds,
- thin_dump contains the thin device,
- thin_rmap resolves mappings correctly,
- the ext4 filesystem is readable,
- but dm-thin still refuses to instantiate the thin device with
Is there any way to
- manually activate Device ID 1,
- create a read-only thin device,
- or reconstruct the thin LV from the existing metadata?
Any ideas would be greatly appreciated. Thanks!