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

To: Steve Dobbelstein <steved@xxxxxxxxxx>
Subject: Re: [Xen-devel] Shouldn't backend devices for VMX domain disks be opened with O_DIRECT?
From: "Philip R. Auld" <pauld@xxxxxxxxxxx>
Date: Thu, 2 Feb 2006 17:41:06 -0500
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Rumor has it that on Thu, Feb 02, 2006 at 04:28:37PM -0600 Steve Dobbelstein 
> 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.



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

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