Very high load when running proxmox backup restore inside a VM

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Andreas_H

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Jan 6, 2021
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Hello,

I have a physical machine running Debian 12 which is backed up to a proxmox backup server using proxmox-backup-client (file-level backup). I want to restore this backup to a VM running on a Proxmox host. I created a Debian 12 VM (full KVM, no container) and installed the backup client.

Soon after starting the restore, load on the proxmox host goes through the roof. I've seen values over 50 before I decided to cancel the restore. The host becomes very sluggish and sometimes even other VMs die.

I don't understand how this happens - the host is a 20-core Xeon Gold 5218R with 128 GB RAM, the VM has 2 cores with 4 GB RAM. VM storage is on local ZFS, which consists of two NVMe SSDs in a ZFS mirror. The backup server is on the same LAN, connected with 1GBit/s.

I tried throttling the restore to ~40MB/s, but that didn't change the problem.

Any ideas what's going on?
 
that sounds like the I/O from the restore brings your storage to its knees.. how is the VM and storage configured?
 
The guest VM has a VirtIO SCSI single controller and two VirtIo SCSI disks, both stored on local ZFS. "Discard" and "IOThread" for both disks are on.

Maybe I made a mistake here, I am not sure I got the concept of VirtIO SCSI single right. I read that this means that each disk would get its own SCSI controller, but I assumed that this would be handled "under the hood". Do I need to manually create a separate controller for each disk?
 
no, that is okay. could you post the full VM config here? what about monitoring besides the "load" value, which doesn't really say much.. how busy are the disk, what does ZFS say, what about memory usage?
 
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