https://wiki.karlsbakk.net/index.php?action=history&feed=atom&title=Libvirt_TRIM%2FUNMAPLibvirt TRIM/UNMAP - Revision history2026-06-14T20:19:28ZRevision history for this page on the wikiMediaWiki 1.44.0https://wiki.karlsbakk.net/index.php?title=Libvirt_TRIM/UNMAP&diff=9&oldid=prevRoy: Moved from https://wiki.malinux.no/index.php/Libvirt_TRIM/UNMAP2020-11-17T18:49:41Z<p>Moved from https://wiki.malinux.no/index.php/Libvirt_TRIM/UNMAP</p>
<p><b>New page</b></p><div>KVM/Libvirt supports UNMAP, similar to TRIM. That way you can free space used by a VM on a qemu2 vdisk not currently in use. <br />
<br />
First of all, your disk images need to be in QCOW2 format. If they are raw, [https://unixblogger.com/2016/06/13/convert-img-raw-to-qcow2/ convert] them first. To allow for deallocation of unused space, you'll need to use UNMAP, similar to TRIM, except that the latter is the ATA counterpart to SCSI's UNMAP. KVM/Libvirt doesn't support the ATA version, so we'll change the driver to SCSI. Usually drives are added as '''VirtIO'''. If they are, change this, using virt-manager. Obviously, you can do this on the commandline, but I'm lazy so I just use the GUI. On my box (Debian Jessie), I can't change '''VirtIO''' to '''VirtIO SCSI''', so I'll have to remove and re-add the drives. If your version of KVM/Libvirt support changing this, skip steps 2 and 3 and just change it. Otherwise, do as follows<br />
<br />
# Shut down the VM<br />
# Start virt-manager and remove the disk(s) from the VM<br />
# Add them in the same order, but as ''VirtIO SCSI''<br />
<br />
Now, you need to add a flag to the driver to make it support UNMAP. Changing the XML config file itself may not work too well, so in my experience, I find it easier to just dump the config, change it, undefine (that is, delete, not touching the data) the old VM and recreating one.<br />
<br />
<pre><br />
virsh dumpxml myvm > myvm.xml<br />
</pre><br />
<br />
Then change myvm.xml, adding the discard part<br />
<br />
<pre><br />
<driver name='qemu' type='qcow2' discard='unmap'/><br />
</pre><br />
<br />
Undefine and redefine the VM<br />
<br />
<pre><br />
virsh undefine myvm<br />
virsh define myvm.xml<br />
</pre><br />
<br />
Start the vm with '''virsh start myvm''' or with virt-manager and wait for it to start. When done, try '''du -sh /var/lib/libvirt/images/myvm.qcow2''' and run '''fstrim /''' inside the VM and check the '''du''' output again. If there are a lot of wasted space, it should be freed by now.<br />
<br />
[mailto:roy@karlsbakk.net roy]</div>Roy