Windows VM on Linux faster than on Native machine !!!! (same hardware)

jimbo45 · Oct 1, 2025

Hi folks
There must be something in the way NTFS works compared with Linux file systems as I'm getting better performance out of a Windows VM running under KVM/QEMU than the same version running on native Windows. Same applications on each - the Windows VM and the physical machine.

The only difference I can see is by using the virtio drivers for running the Windows disks as virtio and the NIC as virtio improves performance quite a bit -- plus I haven't even done the extra bit which really gives the thing a boost is to pass thru a graphics card directly to the VM.

I suspect its the efficiency of the Linux I/O drivers - particularly with Nvme type ssd's rather than the windows NTFS file system which is now donkeys years old.

I haven't done any actual measurements but just using the system seems to work "snappier" -- I'm not a gamer so obviously can't "stress test" a system that way - but things like photoshop and ripping CD's / DVD's work just fine. Even running VLC on a VM with 4k UKD HEVC files works without issue.

I know there's a Windows file system (new) called something like Refs or similar -- not tried that yet --- anybody have experience of that?.

Cheers
jimbo

neves · Oct 1, 2025

It's only natural. If you pass through a GPU (VFIO), you’re basically running near-native Windows - only with a better I/O stack.

Disk I/O Stack under native Windows = NTFS + Windows storage drivers (older design, lots of legacy layers)

vs

Disk I/O under Linux Host (the distribution in question can also have a saying) + KVM Guest = ext4/XFS/btrfs on Linux + virtio-blk/SCSI/NVMe (very lite, paravirtualized)

Not to mention, Native Windows I/O scheduler tends to be rather conservative (adheres to the philosophy: since SSDs are incredibly fast, the best I/O scheduler is no scheduler at all - or one that does minimal work) - while Linux I/O scheduler aggressively reorders and merges requests. And that works rather well for SSDs - which are highly parallel devices that perform best when they have a deep queue of requests to work on simultaneously.

Last but not least... NTFS is heavily metadata-centric, meaning... it keeps almost everything in structured records inside its Master File Table (MFT). Even tiny files can be fully stored inside metadata entries instead of separate disk blocks. On paper probbaly seemed like a great ideea - but it's quite expensive in terms of lookups and write amplification.

jimbo45 · Oct 1, 2025

neves said:
It's only natural. If you pass through a GPU (VFIO), you’re basically running near-native Windows - only with a better I/O stack.

Disk I/O Stack under native Windows = NTFS + Windows storage drivers (older design, lots of legacy layers)

vs

Disk I/O under Linux Host (the distribution in question can also have a saying) + KVM Guest = ext4/XFS/btrfs on Linux + virtio-blk/SCSI/NVMe (very lite, paravirtualized)

Not to mention, Native Windows I/O scheduler tends to be rather conservative (adheres to the philosophy: since SSDs are incredibly fast, the best I/O scheduler is no scheduler at all - or one that does minimal work) - while Linux I/O scheduler aggressively reorders and merges requests. And that works rather well for SSDs - which are highly parallel devices that perform best when they have a deep queue of requests to work on simultaneously.

Last but not least... NTFS is heavily metadata-centric, meaning... it keeps almost everything in structured records inside its Master File Table (MFT). Even tiny files can be fully stored inside metadata entries instead of separate disk blocks. On paper probbaly seemed like a great ideea - but it's quite expensive in terms of lookups and write amplification.

Thanks for the info -- I'm never to old to learn this stuff !!!.

Note though to pass through things like GPU cards etc you need to set things lile IOMMAX or equivalent on either in BIOS or in the Linux kernel start up stuff.

HYPER-V on Windows native machines is also pretty good too but much (IMHO) too inflexible. Without Dynamic USB re-direct (means USB devices need to be defined at boot) or being able to add things like non USB disks / pcie devices etc makes the whole thing a lot more not "user friendly or perhaps more "User aggressive".

Also I do like the fact that one can log out of Linux while leaving a Windows (or other) VM available to other users while if you log off from Windows the VM is inacessible to other users -- this is especially true for vm software like vmware workstation or oracle's virtual box which run in the users work space and aren't type 1 Hypervisors.

But again @neves thanks for the info. !!!

Here's the VM I'm running : All working 100% perfectly.

Cheers
jimbo

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Windows VM on Linux faster than on Native machine !!!! (same hardware)

jimbo45

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neves

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