Understanding storage requirements for snapshots

[KoenDG]

Reading these articles: https://pve.proxmox.com/wiki/Live_Snapshots and https://pve.proxmox.com/wiki/Storage

Basically tells me "snapshots is something you can do" but nothing more.

Recently I tried doing a snapshot and got this as output:

snapshotting 'drive-scsi1' (local-lvm:vm-206-disk-1)
WARNING: You have not turned on protection against thin pools running out of space.
WARNING: Set activation/thin_pool_autoextend_threshold below 100 to trigger automatic extension of thin pools before they get full.
Logical volume "snap_vm-206-disk-1_testsnap" created.
WARNING: Sum of all thin volume sizes (<1.63 TiB) exceeds the size of thin pool pve/data and the amount of free space in volume group (<16.38 GiB).
TASK OK

If I check with lvs I do see it in the output:

snap_vm-206-disk-1_testsnap pve Vri---tz-k 96.00g data vm-206-disk-1

On other clusters, I see the snapshot only taking, say, 4GB on a disk that's 64GB. And here I'm seeing 96GB of 96GB taken, the full 100%. Or am I mistaken?


When looking at my LVM, I see this:


And LVM thin says this:


It's not exactly clear to me why I'm seeing a warning about thin pools running out. Also, it's not clear to me on what storage this snapshot is taking space and if it's 96GB(the full disk size) or not.


What exactly would be some good steps here to learn about all this? And visualize it somewhere in the web GUI? I'm on version 7.4-3

 

[bbgeek17]

If you have a data set of 100GB of actual written data, then take a snapshot, and then go and change every single bit of your existing data - the snapshot size will be 100GB, ie 100% of your user data. If you dont change anything, then the snapshot size will only be a few megabytes of metadata.
As for the warning, it happens when you overprovision your storage capacity. Its there because running out of physical space will be catastrophic for your data:
https://pve.proxmox.com/wiki/LVM2#R... issues-,Thin Overprovisioning,-In a LVM_thin

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Blockbridge : Ultra low latency all-NVME shared storage for Proxmox - https://www.blockbridge.com/proxmox

 

[KoenDG]

So am I to understand that creating a snapshot implies that what is created is a constantly evolving diff with the disk?

Isn't a snapshot supposed to be the state of the machine at a point in time, that can then be gone back to?

 

[shanreich]

So am I to understand that creating a snapshot implies that what is created is a constantly evolving diff with the disk?

Kinda, it is copy-on-write. You can read more about it in the SUSE documentation [1]

Isn't a snapshot supposed to be the state of the machine at a point in time, that can then be gone back to?

It is effectively, but doing it this way has a few upsides: Snapshots can be done (almost) immediately, and they use up a lot less space unless you overwrite large amounts of data. A downside can be that this degrades write performance.


[1] https://documentation.suse.com/sles/12-SP4/html/SLES-all/cha-lvm-snapshots.html

 

[KoenDG]

Alright, thanks.

I'll go have a look at "snapshot mode" in backups.

 

[shanreich]

I'll go have a look at "snapshot mode" in backups.

Be aware that the snapshot mode does not use any storage-level snapshotting when backupping VMs. (it does for containers though). You can read more about this in our wiki article regarding backups [1].

[1] https://pve.proxmox.com/wiki/Backup_and_Restore#_backup_modes

 

[SlimTom]

Can I select which disk of a VM should be in a snaphsot? I would like to snapshot only system disk, but not the "data" disk...
Can I select the location of the snapshot or it is always alongside VM?

Reason for asking:

 

[bbgeek17]

https://forum.proxmox.com/threads/exclude-disk-from-snapshot.44257/
https://forum.proxmox.com/threads/is-there-a-way-to-exclude-drives-from-snapshots.96930/

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Blockbridge : Ultra low latency all-NVME shared storage for Proxmox - https://www.blockbridge.com/proxmox