WindowsServer2025 slow performace

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shady79

New Member
Nov 19, 2025
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Hi,
Some time ago, I installed Windows Server 2025 on Proxmox 8.4.0. It worked quite well until a certain point. However, a few days ago, I noticed that something was wrong. The system started to run very slowly, and it looks like the CPU load is extremely high.
I have the following configuration:

root@pve:/etc/pve/qemu-server# cat 100.conf
agent: 1
bios: ovmf
boot: order=ide0;ide2;net0
cores: 4
cpu: x86-64-v4
efidisk0: local-zfs:vm-100-disk-0,efitype=4m,pre-enrolled-keys=1,size=1M
ide0: local-zfs:vm-100-disk-1,backup=0,cache=writeback,size=512G
ide2: local:iso/virtio-win-0.1.285.iso,media=cdrom,size=771138K
machine: pc-q35-9.2+pve1,viommu=virtio
memory: 8096
meta: creation-qemu=9.2.0,ctime=1752233332
name: WinSerwer2025
net0: virtio=BC:24:11:5D:40:8F,bridge=vmbr0,firewall=1
numa: 1
onboot: 1
ostype: win11
scsihw: virtio-scsi-single
smbios1: uuid=f33437a4-3590-42d6-bb46-19997c117685
sockets: 1
tpmstate0: local-zfs:vm-100-disk-2,size=4M,version=v2.0
vcpus: 4
vga: virtio

The disk performance test is poor.

> Disk Random 16.0 Read 11.13 MB/s 5.4
> Disk Sequential 64.0 Read 59.82 MB/s 5.7
> Disk Sequential 64.0 Write 78.59 MB/s 6.2

I tested it on the virtualizer side,
root@pve:~# hdparm -tT /dev/sda3

/dev/sda3:
Timing cached reads: 1938 MB in 2.00 seconds = 970.93 MB/sec
Timing buffered disk reads: 478 MB in 3.01 seconds = 158.99 MB/sec

I think it's okay.

regards.
Jakub
 
Last edited:
It’s not surprising that you’re seeing poor performance with this setup—your VM disk is using IDE emulation, which is extremely slow under QEMU/KVM. IDE provides no NCQ, no modern I/O scheduling, and significantly higher virtualization overhead compared to VirtIO-SCSI or NVMe. It’s only meant for legacy OSes like Windows XP/2003 or certain rescue situations.

For modern Windows Server versions (2022 / 2025), you should switch the system disk to one of the following:

scsihw: virtio-scsi-single
(scsi0: <your pool>:vm-XYZ-disk-0, discard=on, iothread=1, ssd=1)

You’re also running the VM on local-zfs, which means performance depends heavily on how your pool is configured (mirror vs RAID-Z, volblocksize, SLOG, etc.). Windows VMs on ZFS typically perform best with:
  • volblocksize=64K for system disks and general workloads
  • Special devices / fast SSDs if sync writes matter
  • Avoiding RAID-Z for random I/O workloads
Running IDE on top of ZFS makes latency even worse, because IDE forces synchronous behavior without leveraging ZFS optimizations.
 
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If I do “detach” and then “retach” on the scsii controller, will the system start up?
I have all the drivers installed from virtio-win-0.1.285.iso.
 
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