Re: [Qemu-devel] qemu 2.0, deadlock in block-commit

[Marcin Gibuła]

> Two options for making progress on this bug:
>
> 1. Debug bdrv_drain_all() and find out whether there are any I/O
>     requests remaining.

Yes, there is one request pending on active layer of disk that is being 
commited (on bs->tracked_requests list). IO threads die off because they 
have nothing to do... it seems that requests are somehow not commited 
into threads.

I tried hard (and will continue to try) to debug this, but documentation 
is limited :-) so ANY tips where to look are welcome.

> 2. Post steps for reproducing this problem (exact command-lines or virsh
>     commands used).

I'm using application that talks with libvirt via API, so I describe 
what it does.

1. Create a VM, boot a system. I'm using iso from http://www.sysresccd.org

2. VM has a mounted QCOW2 disk with following hierarchy:
[file1] -> [file2 (active)]

Both are qcow2 files.

3. Open console. Start command:

while true; do dd if=/dev/zero of=/dev/vdX bs=512k oflag=direct; done;

Where vdX is of course qcow2 disk described above.

4. Create snapshot of file2 (virDomainSnapshotCreateXML). So now we have:

[file1] -> [file2] -> [file3 (active)] -

5. Wait couple of seconds (so snapshot fills up).

6. Commit file2 into file1 (virDomainBlockCommit).

7. During commit, another threads is using virDomainGetBlockJobInfo() to 
query its progress.

Note - it doesn't always happen. I have about 1 per 10 failure rate with 
this procedure.

Do you want me to reproduce it manually with pure virsh?

--
mg