Re: Performance on QEMU IDE disks

[Steve Ofsthun <sofsthun@virtualiron.com>]

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
-- 
Steve Ofsthun - Virtual Iron Software, Inc.