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Re: [Xen-devel] Performance on QEMU IDE disks

P M, Priya (STSD) wrote:

We have disabled the Write Cache using sdparm and hdparm utilities. But if you see the results in Linux Full Virtualization, the Sequential Write IOPs are better than Native performance. I am wondering how is this possible? Is there anywhere the caching is happening in the full virtualization case? If you have an idea about this behavior, please let me know.

QEMU does indeed use the buffer cache in Domain 0.  In doing so, it also
will take advantage of Domain 0 read ahead/write behind.  The down side is
that the disk write ordering guarantees expected by DomU filesystems are
violated as well.  If XEN or Domain 0 crashes, your DomU filesystems may
be toast.

To fix this you would need to patch QEMU to used O_DIRECT when accessing
the virtual disk backing store object (block device, file, etc).  We are
currently testing just such a patch for the old QEMU device model.  I
haven't looked at the new device model to see if it already handles this.

Steve Ofsthun - Virtual Iron Software, Inc.

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