While checking out some logs and google search analytics, I found that my post about Discard (TRIM) with KVM Virtual Machines has been referenced far more than I expected it to be. I decided to take this opportitnity to fact-check and correct that article.

I migrated my server from a 10-year old Dual Xeon X5650 to a significantly more performant Ryzen 1600. I undertook the process of migrating and updating my VMs, ensuring the libvirt definitions were current, switching from NFS to 9p, etc.

virtio vs virtio-scsi

My previous post made this observation:

All of my VMs were using virtio disks. However, they don’t pass discard through. However, the virtio-scsi controller does.

It appears that is no longer entirely true. At some point between October 2015 and March 2020 (when I’m writing this), standard virtio-blk devices gained discard support. Indeed, virtio-blk devices actually support discard out of the box, with no additional configuration required:

[guest] $ sudo lsblk -o NAME,MOUNTPOINT,DISC-MAX,FSTYPE
NAME   MOUNTPOINT      DISC-MAX FSTYPE
vdb                          2G 
└─vdb1 [SWAP]                2G swap
vda                          2G 
└─vda1 /                     2G xfs

My research shows this was qemu 4.0 that added this. Fedora 31 has qemu 4.1.

It is no longer required to convert to virtio-scsi.

FWIW, I’m using machine type ‘pc-q35-4.1’. I expect this behaviour is only exposed on machines >= 4.0.

Discard vs unmap/shrink

While the guest will support discards now, that doesn’t actually shrink the disks on the host. I haven’t performed any testing to confirm, but I suspect it is actually performing a passthrough to the underlying storage layer, allowing the SSD to do a proper Garbage Collection, without modifying the file on the host filesystem. That’s all well and good, and a perfectly sane default, but I do still want to shrink files on the host. These VMs are on a somewhat smaller drive, and I wish to reclaim space whenever possible.

To shrink the actual files on the host, the discard mode must be changed to ‘unmap’.

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

This can be done with the same sed-magic used in my last post. Additionally, the option is actually exposed through virt-manager now:

Enable fstrim

As before, use fstrim to issue discards on-demand

$ sudo fstrim -av

Going forward, you can either add ‘discard’ to the mount options in fstab (which will probably hurt performance), or use fstrim periodically. I opted for fstrim, as it has a systemd timer unit that can be scheduled:

$ sudo systemctl enable --now fstrim.timer