Changing default cache mode for new hard disks

J

Julen

Member
Aug 30, 2017
7
0
6
Bremen, Germany
larrucea.eu
I recently learned that using a glusterfs storage backend requires the virtual hard disks to have some cache mode other than "No cache".
Is there any way to overwrite the default PVE settings to ensure that every new hard disk is created with cache mode (i.e.) "Write through" instead of (default) "No cache"?

Thanks in advance
Julen
 
Because of my experience...
I built the glusterfs backend, tried to live-migrate the volumes from NFS to the glustrefs storage, but didn't work.
So, I migrated the volumes off-line, but I couldn't start the VMs because of some error like this:

task started by HA resource agent
kvm: -drive file=gluster://11.22.33.44/pm/images/201/vm-201-disk-0.qcow2,if=none,id=drive- scsi0,format=qcow2,cache=none,aio=native,detect-zeroes=on: Could not read qcow2 header: Invalid argument
TASK ERROR: start failed: command '/usr/bin/kvm -id 201 -name vt-oe412 -chardev 'socket,id=qmp,path=/var/run/qemu-server/201.qmp,server,nowait' -mon 'chardev=qmp,mode=control' -pidfile /var/run/qemu-server/201.pid -daemonize -smbios 'type=1,uuid=012ab6e5-f4b4-12ab-ab12-adf81689cb312' -smp '2,sockets=1,cores=2,maxcpus=2' -nodefaults -boot 'menu=on,strict=on,reboot-timeout=1000,splash=/usr/share/qemu-server/bootsplash.jpg' -vga std -vnc unix:/var/run/qemu-server/201.vnc,x509,password -cpu kvm64,+lahf_lm,+sep,+kvm_pv_unhalt,+kvm_pv_eoi,enforce -m 2014 -k de -device 'pci-bridge,id=pci.2,chassis_nr=2,bus=pci.0,addr=0x1f' -device 'pci-bridge,id=pci.1,chassis_nr=1,bus=pci.0,addr=0x1e' -device 'piix3-usb-uhci,id=uhci,bus=pci.0,addr=0x1.0x2' -device 'usb-tablet,id=tablet,bus=uhci.0,port=1' -device 'virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x3' -iscsi 'initiator-name=iqn.1993-08.org.debian:01:2322b12dacb1' -drive 'if=none,id=drive-ide2,media=cdrom,aio=threads' -device 'ide-cd,bus=ide.1,unit=0,drive=drive-ide2,id=ide2,bootindex=200' -device 'virtio-scsi-pci,id=scsihw0,bus=pci.0,addr=0x5' -drive 'file=gluster://11.22.33.44/pm/images/201/vm-201-disk-0.qcow2,if=none,id=drive-scsi0,format=qcow2,cache=none,aio=native,detect-zeroes=on' -device 'scsi-hd,bus=scsihw0.0,channel=0,scsi-id=0,lun=0,drive=drive-scsi0,id=scsi0,bootindex=100' -netdev 'type=tap,id=net0,ifname=tap201i0,script=/var/lib/qemu-server/pve-bridge,downscript=/var/lib/qemu-server/pve-bridgedown,vhost=on' -device 'virtio-net-pci,mac=AB:CD:FG:HI:JK:LM,netdev=net0,bus=pci.0,addr=0x12,id=net0,bootindex=300'' failed: exit code 1

The only way to make it work, was to set another cache mode. Now it works and I can even live migrate between storage back-ends.
Still, it would be nice to have the default cache mode changed, so that my clients don't have to struggle to figure out why their new VM is not starting.
 
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