Red Hat Bugzilla – Bug 294191
Causes read errors on CD/DVD drive
Last modified: 2008-06-16 22:25:38 EDT
Description of problem:
I believe KVM or QEMU fails to correctly arbitrate access to the host's CD/DVD
drive, and can cause read errors on the drive.
I found that if I read from DVD drive on the host that is also assigned as a
device for a Windows XP guest, that I can get read errors (corrupted data).
The errors are very reproducible. I was suspecting a hardware fault.
However, I decided to stop libvirtd and try reading the drive again. No errors!
So, whereas this isn't definitive proof of a problem in KVM, that is certainly
where the evidence is pointing.
Version-Release number of selected component (if applicable):
Steps to Reproduce:
1. Generate a checksum for a large file (typically 1GB+) on a DVD.
2. Load the DVD into a drive that's also allocated to a guest Windows XP
(possibly other OSes too).
3. On the host, generate the checksum again.
4. You will get a read error.
5. Switch off the host and shutdown libvirtd.
6. Generate the checksum - no error.
Read errors/data corruption.
kvm should correctly arbitrate access to the DVD media so that host and guest
can access the data correctly or not at all.
Further tests provided mixed results.
It's not as reproducible as it first appeared.
Also, it might be related to burning a DVD on the host.
DVD drive 1 (internal): burn DVD
DVD drive 1 (internal): read errors reading the DVD
DVD drive 2 (external): no read errors reading the DVD
I've had several DVD burns appear to be corrupted until I switched off the
CD/DVD drive in the guest.
After switching off the CD/DVD drive in the guest, the DVD reads became reliable.
They remained reliable even after switching the CD/DVD drive back on in the guest.
I'm pretty confused by this evidence, but it does seem to be related to KVM or QEMU.
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