Hello, I am running several qemu-kvm VM servers on Ubuntu 12.04 with qemu-kvm 1.4.0. Most of the guests are also running Ubuntu 12.04. I am using qcow2 disk images with the virtio driver in almost all cases, and am storing the disks on two types of storage devices: * a shared NFS server mounted on each host * local ext4 filesystems on each host (on top of an md RAID1 of HDDs)
It seems like the consensus for NFS is to use cache=none in order to support live migration between hosts. For local ext4 filesystems, I see from this message and elsewhere that cache=writeback is now the default disk cache mode, and is safe on ext4: http://lists.gnu.org/archive/html/qemu-devel/2012-02/msg02689.html All of the hosts and guests are using ext4 (with the default data=ordered,barrier). Thus it seems probable that the kernel (3.2.x or newer) should support flushing correctly? Is there a way to check in the guest is utilizing WCE (or any other required components) correctly? I recently experienced UPS failure on several hosts which caused a hard shutdown. After restarting, 3 of the guests had corruption on their disks and required a fairly long fsck to fix. Afterwards, data that had been written to the disks several hours before the crash was corrupted, which makes me think that it was never fsync()-ed to the non-volatile storage. Is it safe in this setup to use cache=writeback? Or, should I use cache=writethrough instead? Thanks, Andrew Martin
