[SOLVED] Mount a Ceph snapshot in a running LXC container

[lucaferr]

Hi!
I have a big LXC container (2 TB) running on a Proxmox VE 5.4 with Ceph Luminous storage. The container is running mission critical services and cannot be stopped. Two days ago I took a snapshot from the Proxmox interface (you can see it named "pre_upgrade_tls" in the screenshot below).
Now, since by mistake I delete a folder, I would need to recover that folder from the snapshot...but I can't rollback the whole container: I just need that single folder.
Is it possible to mount the snapshot read-only somewhere, so I can rsync the folder I need to the running LXC container? Is this operation safe? (if it's not, I already have a plan B which is to uncompress the backups, but it's going to take several hours). As I mentioned, the container must continue to run and cannot be rebooted.
Thank you very much!

 

[Alwin]

Is it possible to mount the snapshot read-only somewhere, so I can rsync the folder I need to the running LXC container?

You will need to map (read-only) the snapshot on the Proxmox VE node and then copy the folder into the container.

Is this operation safe? (if it's not, I already have a plan B which is to uncompress the backups, but it's going to take several hours).

Safe in what way?

And last but not least Proxmox VE 5.4 is long-time EoL.

 

[lucaferr]

You will need to map (read-only) the snapshot on the Proxmox VE node and then copy the folder into the container.

Thank you Alwin! Could you please tell me the steps to map in read-only on the Proxmox VE node?

Safe in what way?

I think you already answered "yes, this is safe". It was just a warning not to point me to an emergency hack if it involved any risk, since I have a plan B

And last but not least Proxmox VE 5.4 is long-time EoL.

Yep, you're perfectly right, upgrade to 6.3 and latest Ceph is planned for 2021

 

[Alwin]

Thank you Alwin! Could you please tell me the steps to map in read-only on the Proxmox VE node?

rbd map --read-only <pool>/<snapshot> and snapshots you get by rbd ls -l.

 

[lucaferr]

Many thanks, it worked! Here the steps that I followed, for anyone else interested:

1. rbd ls -l <poolname> (to get the snapshot name)
2. rbd showmapped (just to check that the snapshot is not already mapped)
3. mkdir <mountpoint>
4. rbd map --read-only <poolname>/<diskname>@snapname
5. rbd showmapped (just to verify that the snapshot has been correctly mapped)
6. mount -o noload /dev/rbd/<poolname>/<diskname>@snapname <mountpoint>
   (if it's a VM instead of a CT, you must also add partition ("-part1" after @snapname, if you want to mount the first partition)
7. Mounted in read-only! Do what you need to do...
8. umount <mountpoint>
9. rbd unmap <poolname>/<diskname>@snapname
10. rbd showmapped (just to verify that the snapshot is not mapped anymore)

Cheers!