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Re: [libvirt] [PATCH] Extra filesystem options during backend filesystem mount.

On Thu, Jul 16, 2009 at 08:24:09PM +0530, Harshavardhana wrote:
> Hi Daniel,
> On Thu, Jul 16, 2009 at 7:46 PM, Daniel P. Berrange <berrange redhat com>wrote:
> > On Thu, Jul 16, 2009 at 06:06:25AM -0700, Harshavardhana wrote:
> > > New option index added to support -o options for various netfs.
> > > Currently added an option for glusterfs.
> >
> > What effect does it have  ? Or why do we want/need it
> >
> Options could be required for filesystem to have few enhaced handling at the
> site where they will be under use. Correct approach for a configurable will
> be a new "XML" option in this case.
> Regarding current patch:
> This is required for the glusterfs to work properly with VM's.  Right now
> there is a
> problem/difficulty in using direct-io based mechanism in the fuse kernel
> module
> when used with "XEN" in its "tap:aio" framework, we have seen xen vms hang
> over glusterfs or any fuse based filesystem due to fact that fuse module
> doesn't yet support "aio" with O_DIRECT internally as a kernel module. To
> have a work around fix we have to hardcode this value due to its usage in
> case of VM's.
> We are currently fixing this problem by fixing directly O_DIRECT problem in
> fuse. Which will be available in later releases for kernel.

ACK, I see why you need this for the current releases of kernel/glusterfs.

As & when this problem is fixed we'll need to either remove this, or
provide a way to turn it off again. I don't think this is the kind of
tunable that should be exposed in the XML, since this is really just a
hack to work around a bug in a specific releases. Someone using libvirt
has no way to decide whether the hack is needed or not, so making them
set it in the XML would not be desirable. One possibility would be to
have a config file /etc/libvirt/storage.conf for controlling certain 
options like this per host.

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