Re: [Xen-devel] Shouldn't backend devices for VMX domain disks be opened with O_DIRECT?

["Philip R. Auld" <pauld@xxxxxxxxxxx>]

Hi,

Rumor has it that on Thu, Feb 02, 2006 at 04:28:37PM -0600 Steve Dobbelstein 
said:
> aliguori@xxxxxxxxxxxxxxxxxxxxxxx wrote on 02/02/2006 03:46:11 PM:
> 
> > I would doubt it.  Since it's usually opening a file, and qemu-dm is
> > emulating a contigous disk, you probably want the buffer cache to
> > reorder events.
> 
> I guess we're not usual since our backend is an LVM volume. :)
> 
> I can appreciate how writing to the buffer cache can speed up the response
> to the I/O and make it more efficient in its writing to the backend device
> by reordering events.  However, I'm still wondering if we have a data
> corruption issue should dom0 crash before it writes the data in the buffer
> cache to disk, data that the domain expects to be on the disk but won't be
> there when the domain is restarted.

I agree. It sounds like a correctness problem. It's just like disks
with write caching enabled.  

> 
> > Are you seeing a performance improvement?  Should be easy to check.
> 

It's more about correctness and data integrity than performance.


Cheers,

Phil


> We just started doing the first runs of disk performance tests when we
> noticed this behavior and thought we should bring it up on the list.  We
> don't have enough data points to compare yet.  We'll post problems/issues
> if/when we find them.
> 
> Steve D.
> 
> 
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@xxxxxxxxxxxxxxxxxxx
> http://lists.xensource.com/xen-devel

-- 
Philip R. Auld, Ph.D.                          Egenera, Inc.    
Software Architect                            165 Forest St.
(508) 858-2628                            Marlboro, MA 01752

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel