[Qemu-devel] Using cache=writeback safely on qemu 1.4.0 and later

[Andrew Martin]

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