Re: [Libguestfs] [libvirt] Quantifying libvirt errors in launching the libguestfs appliance
On 01/13/2016 05:18 AM, Richard W.M. Jones wrote:
As people may know, we frequently encounter errors caused by libvirt
when running the libguestfs appliance.
I wanted to find out exactly how frequently these happen and classify
the errors, so I ran the 'virt-df' tool overnight 1700 times. This
tool runs several parallel qemu:///session libvirt connections both
creating a short-lived appliance guest.
Note that I have added Cole's patch to fix https://bugzilla.redhat.com/1271183
"XML-RPC error : Cannot write data: Transport endpoint is not connected"
Results:
The test failed 538 times (32% of the time), which is pretty dismal.
To be fair, virt-df is aggressive about how it launches parallel
libvirt connections. Most other virt-* tools use only a single
libvirt connection and are consequently more reliable.
Of the failures, 518 (96%) were of the form:
process exited while connecting to monitor: qemu: could not load kernel
'/home/rjones/d/libguestfs/tmp/.guestfs-1000/appliance.d/kernel': Permission
denied
which is https://bugzilla.redhat.com/921135 or maybe
https://bugzilla.redhat.com/1269975. It's not clear to me if these
bugs have different causes, but if they do then potentially we're
seeing a mix of both since my test has no way to distinguish them.
I just experimented with this, I think it's the issue I suggested at:
https://bugzilla.redhat.com/show_bug.cgi?id=1269975#c4
I created two VMs, kernel1 and kernel2, just booting off a kernel in
$HOME/session-kernel/vmlinuz. Then I added this patch:
diff --git a/src/qemu/qemu_process.c b/src/qemu/qemu_process.c
index f083f3f..5d9f0fa 100644
--- a/src/qemu/qemu_process.c
+++ b/src/qemu/qemu_process.c
@@ -4901,6 +4901,13 @@ qemuProcessLaunch(virConnectPtr conn,
incoming ? incoming->path : NULL) < 0)
goto cleanup;
+ if (STREQ(vm->def->name, "kernel1")) {
+ for (int z = 0; z < 30; z++) {
+ printf("kernel1: sleeping %d of 30\n", z + 1);
+ sleep(1);
+ }
+ }
+
/* Security manager labeled all devices, therefore
* if any operation from now on fails, we need to ask the caller to
* restore labels.
Which is right after selinux labels are set on VM startup. This is then easy
to reproduce with:
virsh start kernel1 (sleeps)
virsh start kernel2 && virsh destroy kernel2
The shared vmlinuz is reset to user_home_t after kernel2 is shut down, so
kernel1 fails to start after the patch's timeout
When we detect similar issues with <disk> devices, like when the media already
has the expected label, we encode 'relabel=no' in the disk XML, which tells
libvirt not to run restorecon on the disks path when the VM is shutdown.
However kernel/initrd XML doesn't have support for this XML, so it won't work
there. Adding that could be one fix.
But I think there's longer term plans for this type of issue by using ACLs, or
virtlockd or something, Michal had patches but I don't know the specifics.
Unfortunately even hardlinks share selinux labels so I don't think there's any
workaround on the libguestfs side short of using a separate copy of the
appliance kernel for each VM
- Cole