Discard=Unmap on Virtio Disks

[aim60]

I ran across an article whose author claimed that on recent versions of qemu,  discard=unmap was functional on virtio disks.  So I did some testing.

 

I have not run all permutations, but have tested:

  Unraid 6.9.2

  UEFI and Seabios VMs

  Q35-5.1 and i440fx-5.1 VMs

  Windows 10 and Windows 11 with the latest virtio drivers, virtio-win-0.1.196.iso

  Ubuntu 21.4

  raw and qcow2 vdisks

  vdisks on XFS and BTRFS disks

  vdisks with and without the copy-on-write attribute set

 

In all cases, the virtio disk was functionally equivalent to a virtio-scsi disk.  On file deletion, windows unmapped blocks immediately.  Linux unmapped blocks after running fstrim.

 

https://chrisirwin.ca/posts/discard-with-kvm-2020

Edited July 21, 2021 by aim60

[scorcho99]

Thanks for this!

[WillCroPoint]

Thank you so much.

[bobbowitch]

Thank you for this nugget!

[thymon]

Hello,

 

Doesn't work for me

 

Quote

<emulator>/usr/local/sbin/qemu</emulator>
    <disk type='file' device='disk'>
      <driver name='qemu' type='raw' cache='writeback' discard='unmap'/>
      <source file='/mnt/user/domains/Gaming/vdisk1.img'/>
      <target dev='hdc' bus='virtio'/>
      <boot order='1'/>
      <address type='pci' domain='0x0000' bus='0x03' slot='0x00' function='0x0'/>
    </disk>

 

Can you help me ?

Edited May 11 by thymon

[JorgeB]

Did you run the Windows Defrag and Optimize drives after making the change?

[thymon]

Of course...

[JorgeB]

If you have the space copy the vdisk elsewhere, then copy back using cp --sparse=always, that will confirm if the vdisk growing is the problem. 

[thymon]

Ok, I'll try that.
I've free space in the Array.

 

thx