I can't tell if this is a kvm virtio blk device regression, with cache=none and cache=directsync, or if it's a Btrfs regression.
The summary is that on a host using (Fedora) kernel 3.18.9, 3.19.2, or any 4.0.0 kernel, with qcow2 on Btrfs, and either cache=none or directsync, the guest Linux OS experiences many I/O blk device errors. https://bugzilla.redhat.com/show_bug.cgi?id=1204569 If I put the qcow2 on XFS, the problem doesn't happen. If I keep the qcow2 on Btrfs, and change the cache= to writeback, writethrough, or unsafe, the problem doesn't happen. It happens with either qcow2 compat 0.10 or 1.1. Raw files were not tested. And block devices other than virtio were not tested. In the guest, all file systems experience this and complain, some more than others. Btrfs is most tolerant mainly reporting write errors but completes an OS installation; ext4 complains a lot but also completes an OS installation; XFS complains and eventually gives up with a hardware I/O error and the OS installation fails. I did this test maybe two years ago and this combination was safe at that time. -- Chris Murphy -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
