Libvirt TRIM/UNMAP - Revision history

Roy at 00:42, 6 December 2017

2017-12-06T00:42:57Z

← Older revision		Revision as of 00:42, 6 December 2017
Line 1:		Line 1:
	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.		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.

	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		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

	# Shut down the VM		# Shut down the VM

Roy at 00:42, 6 December 2017

2017-12-06T00:42:37Z

← Older revision		Revision as of 00:42, 6 December 2017
Line 1:		Line 1:
	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.		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.

	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		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

	# Shut down the VM		# Shut down the VM

Roy at 00:42, 6 December 2017

2017-12-06T00:42:17Z

← Older revision		Revision as of 00:42, 6 December 2017
Line 1:		Line 1:
	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.		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.

	To ~~do this~~, 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		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

	# Shut down the VM		# Shut down the VM

Roy at 00:32, 6 December 2017

2017-12-06T00:32:57Z

← Older revision		Revision as of 00:32, 6 December 2017
Line 1:		Line 1:
	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.		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.

	To do this, 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		To do this, 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

	# Shut down the VM		# Shut down the VM
Line 26:		Line 26:
	</pre>		</pre>

	Start the vm with ''virsh start ~~mym~~'' 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 ''du'' again. If there are a lot of wasted space, it should be freed by now.		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.

	[mailto:roy@karlsbakk.net roy]		[mailto:roy@karlsbakk.net roy]

Roy: Some info about UNMAP/TRIM on KVM/Libvirt

2017-12-06T00:31:48Z

Some info about UNMAP/TRIM on KVM/Libvirt

New page

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.

To do this, 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

# Shut down the VM
# Start virt-manager and remove the disk(s) from the VM
# Add them in the same order, but as ''VirtIO SCSI''

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.

<pre>
virsh dumpxml myvm > myvm.xml
</pre>

Then change myvm.xml, adding the discard part

<pre>
<driver name='qemu' type='qcow2' discard='unmap'/>
</pre>

Undefine and redefine the VM

<pre>
virsh undefine myvm
virsh define myvm.xml
</pre>

Start the vm with ''virsh start mym'' 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 ''du'' again. If there are a lot of wasted space, it should be freed by now.

[mailto:roy@karlsbakk.net roy]