Christian,
In this case Juju (not charms) uses the following command in its code to
create a VM image based on a common cloud image:
out, err := params.runCmd(
"qemu-img",
"create",
"-b", backingPath,
"-f", "qcow2",
imgPath,
fmt.Sprintf("%dG", params.RootDisk))
This is done when charms are requested to be deployed into a VM provisioned by
a juju machine agent (--to kvm:<host-machine-id> syntax analogous to --to
lxd:<host-machine-id>).
The reported problem specifically occurs when multiple machines are
provisioned on the same machine.
My understanding is:
1) that Redirect-on-Write type of snapshot is safe to do (even if multiple
copies of qemu-img are using the same backing image at the same moment) because
the backing image does not change in progress;
2) it is safe to run multiple VMs using different snapshots that use the same
base image (because of the Redirect-on-Write behavior).
https://wiki.qemu.org/Documentation/CreateSnapshot
I initially thought that it might be a race between multiple parallel
`qemu-img create -b bfile -f qcow2 <targetimg>` invocations but the
errors are present in the libvirt domain log so it looks like I might be
wrong.
Given that each VM has its own snapshot based on the cloud image and its own
ISO, I am not sure why the write lock is necessary:
-drive file=/var/lib/juju/kvm/guests/juju-5ad6c1-6-kvm-0.qcow
-drive file=/var/lib/juju/kvm/guests/juju-5ad6c1-6-kvm-0-ds.iso
Any thoughts about that?
I included `ls -il /var/lib/juju/kvm/guests/` output in comment #2 to
show if there were any hard links (none based on what I see in the first
column).
The patch for snapshots and backing images seems to be included in qemu/bionic
already so the qemu-img -b (...) invocation shouldn't be a problem:
➜ qemu git:(ubuntu/bionic) grep -RiP ' Open backing image in force share mode'
debian/patches/ubuntu/qemu-stable-2.11.1.patch:Subject: [PATCH 09/80] block:
Open backing image in force share mode for size
https://lists.gnu.org/archive/html/qemu-devel/2017-12/msg02816.html
https://git.qemu.org/?p=qemu.git;a=commit;h=cc954f01e3c004aad081aa36736a17e842b80211
git --no-pager tag --contains=cc954f01e3c004aad081aa36736a17e842b80211 | grep
-v rc
v2.12.0
v2.12.1
v3.0.0
v3.1.0
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https://bugs.launchpad.net/bugs/1816170
Title:
KVM instances fail at creation with: Failed to get shared "write" lock
Is another process using the image?
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