One issue is that when the libvirt cache mode is set to "none" what is
passed to qemu is "write-cache=on". If I set libvirt cache mode to
"writethrough" qemu gets "write-cache=off" !
The remaining problem is solving the lock issue on NFS. Came back to
this issue again today and its again blocked everything.
The reason qemu cannot write the lock-file is the user/group it runs as
(libvirt-qemu/kvm) do not exist on the NFS server so there is no
possible NFS mapping for those. So, despite reading the other bugs and
the various links within them I'm still not clear how to simply tell
qemu not to bother with locking at all - I KNOW it is safe, so do what I
damned well tell you!
The qemu log ends with:
2021-11-12T20:35:51.527063Z qemu-system-x86_64: -device ide-
cd,bus=ide.0,unit=0,share-
rw=on,drive=libvirt-1-format,id=ide0-0-0,write-cache=off: Failed to lock
byte 100: No locks available
The relevant lines for the qemu-system-x86_64 command-line from the same
log file are:
-blockdev
'{"node-name":"libvirt-2-format","read-only":true,"discard":"ignore","cache":{"direct":true,"no-flush":false},"driver":"raw","file":"libvirt-2
-storage"}' \
-device
virtio-blk-pci,bus=pci.0,addr=0x7,drive=libvirt-2-format,id=virtio-disk0,bootindex=1,write-cache=on
\
-blockdev
'{"driver":"file","filename":"/srv/NAS/Sunny/Downloads/ISO/kubuntu-21.04-desktop-amd64.iso","node-name":"libvirt-1-storage","cache":{"direct":false,"no-flush":false},"auto-read-only":true,"discard":"unmap"}'
\
I tried adding various lines to:
/etc/apparmor.d/local/abstractions/libvirt-qemu
/srv/NAS/Sunny/Downloads/** rk,
or
/srv/NAS/Sunny/Downloads/**/* rk,
But of course these aren't going to help because they have no effect on
the underlying NFS permissions.
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https://bugs.launchpad.net/bugs/1948525
Title:
Alters Performance options > Cache mode causing start failure
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