Windows partition slow write speeds

[juliannieborg]

Hi all,

I am experiencing a performance issue with a Windows Server 2022 VM in my Proxmox environment.
The VM is configured with a disk that has been split into two partitions. The problem seems to affect only the Windows partition. Here is a breakdown of the issue:

• On the Windows partition, when performing a sequential 8M test, I achieve a read speed of around 5000 MB/s, but the write speed drops to just 100 MB/s.
• On the other partition of the same disk (same drive and configuration), I consistently get read speeds of 5000 MB/s and write speeds of 5000 MB/s, which are the expected results.
• When I add an additional disk to the same VM, it also performs as expected (5000 MB/s read and write speeds).
• When I combine the two partitions, I achieve a read speed of around 5000 MB/s, and the write speed drops to 100 MB/s.

I am unsure why only the Windows partition has significantly lower write speeds compared to the other partition on the same disk.
I have tried various scsi controllers, cache settings, and disk types. I have also tested without network adapters, with ballooning enabled and disabled, and without ballooning drivers.

The drive is a lvm-thin of 12 ssd's in raid 1+0.

VM Configuration:
agent: 1,fstrim_cloned_disks=1
balloon: 0
bios: ovmf
boot: order=virtio0;net0;ide2
cores: 10
cpu: host
efidisk0: StorageMWH04:vm-1513-disk-0,efitype=4m,pre-enrolled-keys=1,size=4M
hotplug: disk,network,usb
ide2: local:iso/virtio-win.iso,media=cdrom,size=708140K
machine: pc-q35-8.1
memory: 32768
meta: creation-qemu=8.1.5,ctime=1723667604
name: Diemex-TS
net0: virtio=BC:24:11:BD:AB:EC,bridge=vmbr1
numa: 0
onboot: 1
ostype: win11
scsihw: virtio-scsi-single
smbios1:
sockets: 1
tpmstate0: StorageMWH04:vm-1513-disk-2,size=4M,version=v2.0
unused0: StorageMWH04:vm-1513-disk-3
vga: clipboard=vnc
virtio0: StorageMWH04:vm-1513-disk-1,cache=unsafe,discard=on,iothread=1,size=300G+


Who has the answer?