On Sat, Jul 06, 2013 at 03:03:07PM +0200, Bernd Schubert wrote:
> On 07/03/2013 10:47 AM, Stefan Hajnoczi wrote:
> > On Tue, Jul 02, 2013 at 10:40:11AM -0400, Brian J. Murrell wrote:
> >> I have a cluster of VMs setup with shared virtio-scsi disks. The
> >> purpose of sharing a disk is that if a VM goes down, another can
> >> pick up and mount the (ext4) filesystem on shared disk a provide
> >> service to it.
> >>
> >> But just to be super clear, only one VM ever has a filesystem
> >> mounted at a time even though multiple VMs technically can access
> >> the device at the same time. A VM mounting a filesystem ensures
> >> absolutely that no other node has it mounted before mounting it.
> >>
> >> That said, what I am finding is that when one a node dies and
> >> another node tries to mount the (ext4) filesystem, it is found dirty
> >> and needs an fsck.
> >>
> >> My understanding is that with ext{3,4}, this should not be the case
> >> and indeed it is my experience, on real hardware with coherent disk
> >> caching (i.e. no non-battery-backed caching disk controllers lying
> >> to the O/S about what has been written to physical disk) that this
> >> is the case. That is, a node failing does not leave an ext{3,4}
> >> filesystem dirty such that it needs an fsck.
> >>
> >> So, clearly, somewhere between the KVM VM and the physical disk,
> >> there is a cache that is resulting in the guest O/S believing data
> >> is being written to physical disk that is not actually being written
> >> there. To that end, I have ensured that on these shared disks that
> >> I set "cache=none", but this does not seem to have fixed the
> >> problem.
> >
> > I expect journal replay and possibly fsck when an ext4 file system was
> > left in a mounted state and with I/O pending (e.g. due to power
> > failure).
> >
> > A few questions:
> >
> > 1. Is the guest mounting the file system with barrier=0? barrier=1 is
> > the default.
> >
> > 2. Do the physical disks have a volatile write cache enabled (if yes,
> > the guest should use barrier=1)? If the physical disks have a
> > non-volatile write cache or the write cache is disabled (then
> > barrier=0 is okay).
>
> Er, why? The As far as I understood Brian the physical disks have not
> been reset, so their cache should be irrelevant?
You are right. The physical disk write cache should not matter in this
case.
Stefan
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