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

To: xen-devel@xxxxxxxxxxxxxxxxxxx
Subject: Re: [Xen-devel] Shouldn't backend devices for VMX domain disks be opened with O_DIRECT?
From: Steve Dobbelstein <steved@xxxxxxxxxx>
Date: Thu, 2 Feb 2006 16:28:37 -0600
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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.

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

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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