Proxmox – Fixing Slow Disk Read/Write Speeds in Windows Guest VM
If you’ve ever deployed a Windows Server guest on Proxmox VE and noticed painfully slow disk performance — especially write speeds — you’re not alone.
In my case, I was running the latest Proxmox 9 with a Windows Server 2025 Guest VM hosted on a Dell SC3020 storage array, connected over Fibre Channel of course the multi-path configured.
Despite having the latest VirtIO drivers installed and using a VirtIO SCSI controller, write performance was extremely poor — to the point where even simple file copies were frustratingly slow.
After much testing, the real breakthrough came from enabling Write Back cache on the virtual disk. This simple setting improved write speeds dramatically and completely resolved the issue.
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