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# Disk performance
Taken from https://forum.proxmox.com/threads/virtio-scsi-vs-virtio-scsi-single.28426/
> VirtIO SCSI vs VirtIO SCSI Single boils down to a simple architectural choice that has real performance implications:
>
> Standard VirtIO SCSI uses one controller that handles up to 16 disks, while Single dedicates one controller per disk. This matters most when using IOThreads (iothread=1), because threads work at the controller level.
>
> When using IOThreads, Single shows significantly better performance (often 30-50% improvement) because each disk gets its own dedicated processing thread. Without IOThreads, the performance difference is minimal (typically less than 5%).
>
> So the choice is straightforward:
>
> > Want maximum disk performance? Use Single + iothread=1
> > Managing lots of disks with limited resources? Standard might be better to avoid thread overhead
> > From the VM's perspective, they work exactly the same
>
> This explains why benchmarks consistently show better I/O performance with Single + iothread=1, while keeping the underlying architectural differences clear.
Which results in the following packer configuration :
```hcl
...
# VM Hard Disk Settings
scsi_controller = "virtio-scsi-single"
disks {
...
io_thread = true
}
...
```
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