Dear mailing list,
I'm using QEMU 4.2.0 (coming with Ubuntu 20.04 LTS) to run a virtual webserver. It uses a ZFS volume as its disk over the VirtIO bus (virtio-blk).
After equipping my server with SSDs and moving my virtual webserver's disk to them (actually a ZFS volume in a mirror configuration), I was surprised to find out that discard commands didn't work. When running fstrim on the guest, I got:
fstrim: /: FITRIM ioctl failed: Operation not supported
This surprised me because I read about virtio-blk being adapted so that it can handle discard commands. I checked the source code for QEMU 4.2.0 and indeed the changes to virtio-blk to support discard were in there.
Switching to the SCSI bus solved the problem. But nevertheless, shouldn't it also be working with virtio-blk now?
Further below, I'm posting the QEMU command lines.
Thanks in advance for any ideas on this.
Best regards
David Scherfgen
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QEMU command lines:
With the VirtIO bus (discard not working):
qemu-system-x86_64 -enable-kvm -name guest=webserver,debug-threads=on -S -object secret,id=masterKey0,format=raw,file=/var/lib/libvirt/qemu/domain-30-webserver/master-key.aes -machine pc-i440fx-4.2,accel=kvm,usb=off,dump-guest-core=off -cpu qemu64 -m 49152 -overcommit mem-lock=off -smp 8,sockets=1,cores=4,threads=2 -uuid 7d619885-9c53-2fa3-ca69-032e6fba6a90 -no-user-config -nodefaults -chardev socket,id=charmonitor,fd=32,server,nowait -mon chardev=charmonitor,id=monitor,mode=control -rtc base=utc -no-shutdown -boot strict=on -device piix3-usb-uhci,id=usb,bus=pci.0,addr=0x1.0x2 -device virtio-scsi-pci,id=scsi0,bus=pci.0,addr=0x5 -blockdev {"driver":"host_device","filename":"/dev/zvol/tank-ssd/webserver","node-name":"libvirt-1-storage","cache":{"direct":true,"no-flush":false},"auto-read-only":true,"discard":"unmap"} -blockdev {"node-name":"libvirt-1-format","read-only":false,"discard":"unmap","cache":{"direct":true,"no-flush":false},"driver":"raw","file":"libvirt-1-storage"} -device virtio-blk-pci,scsi=off,bus=pci.0,addr=0x6,drive=libvirt-1-format,id=virtio-disk0,bootindex=1,write-cache=on -netdev tap,fd=34,id=hostnet0,vhost=on,vhostfd=35 -device virtio-net-pci,netdev=hostnet0,id=net0,mac=00:50:56:00:f2:8e,bus=pci.0,addr=0x3 -vnc
0.0.0.0:10000,password -device cirrus-vga,id=video0,bus=pci.0,addr=0x2 -device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x4 -cpu host -k de -sandbox on,obsolete=deny,elevateprivileges=deny,spawn=deny,resourcecontrol=deny -msg timestamp=on
With the SCSI (discard working):
qemu-system-x86_64 -enable-kvm -name guest=webserver,debug-threads=on -S -object secret,id=masterKey0,format=raw,file=/var/lib/libvirt/qemu/domain-31-webserver/master-key.aes -machine pc-i440fx-4.2,accel=kvm,usb=off,dump-guest-core=off -cpu qemu64 -m 49152 -overcommit mem-lock=off -smp 8,sockets=1,cores=4,threads=2 -uuid 7d619885-9c53-2fa3-ca69-032e6fba6a90 -no-user-config -nodefaults -chardev socket,id=charmonitor,fd=32,server,nowait -mon chardev=charmonitor,id=monitor,mode=control -rtc base=utc -no-shutdown -boot strict=on -device piix3-usb-uhci,id=usb,bus=pci.0,addr=0x1.0x2 -device virtio-scsi-pci,id=scsi0,bus=pci.0,addr=0x5 -blockdev {"driver":"host_device","filename":"/dev/zvol/tank-ssd/webserver","node-name":"libvirt-1-storage","cache":{"direct":true,"no-flush":false},"auto-read-only":true,"discard":"unmap"} -blockdev {"node-name":"libvirt-1-format","read-only":false,"discard":"unmap","cache":{"direct":true,"no-flush":false},"driver":"raw","file":"libvirt-1-storage"} -device scsi-hd,bus=scsi0.0,channel=0,scsi-id=0,lun=0,device_id=drive-scsi0-0-0-0,drive=libvirt-1-format,id=scsi0-0-0-0,bootindex=1,write-cache=on -netdev tap,fd=34,id=hostnet0,vhost=on,vhostfd=35 -device virtio-net-pci,netdev=hostnet0,id=net0,mac=00:50:56:00:f2:8e,bus=pci.0,addr=0x3 -vnc
0.0.0.0:10000,password -device cirrus-vga,id=video0,bus=pci.0,addr=0x2 -device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x4 -cpu host -k de -sandbox on,obsolete=deny,elevateprivileges=deny,spawn=deny,resourcecontrol=deny -msg timestamp=on