On 05/11/2010 11:32 AM, Jamie Lokier wrote:
Anthony Liguori wrote:
On 05/11/2010 08:12 AM, Paul Brook wrote:
cache=always (or a more scary name like cache=lie to defend against
idiots)
Reads and writes are cached. Guest flushes are ignored. Useful for
dumb guests in non-critical environments.
I really don't believe that we should support a cache=lie. There are
many other obtain the same results. For instance, mount your guest
filesystem with barrier=0.
Ideally yes. However in practice I suspect this is still a useful option.
Is
it even possible to disable barriers in all cases (e.g. NTFS under
windows)?
In a production environment it's probably not so useful - you're generally
dealing with long lived, custom configured guests.
In a development environment the rules can be a bit different. For example
if
you're testing an OS installer then you really don't want to be passing
magic
mount options. If the host machine dies then you don't care about the
state of
the guest because you're going to start from scratch anyway.
Then create a mount point on your host and mount the host file system
under that mount with barrier=0.
Two reasons that advice doesn't work:
1. It doesn't work in many environments. You can't mount a filesystem
with barrier=0 in one place and barrier=1 on a different point, and
there's ofen only one host partition.
qemu-img create -f raw foo.img 10G
mkfs.ext3 foo.img
mount -oloop,rw,barrier=1 -t ext3 foo.img mnt
Works perfectly fine.
2. barrier=0 does _not_ provide the cache=off behaviour. It only
disables barriers; it does not prevent writing to the disk hardware.
The proposal has nothing to do with cache=off.
The problem with options added for developers is that those options are
very often accidentally used for production.
We already have risky cache= options. Also, do we call fdatasync
(with barrier) on _every_ write for guests which disable the
emulated disk cache?
None of our cache= options should result in data corruption on power
loss. If they do, it's a bug.
Regards,
Anthony Liguori
-- Jamie
