Bug 444047 - F9 iozone w/ direct I/O performing higher than the raw device
F9 iozone w/ direct I/O performing higher than the raw device
Product: Fedora
Classification: Fedora
Component: libvirt (Show other bugs)
x86_64 Linux
high Severity low
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Assigned To: Daniel Veillard
Fedora Extras Quality Assurance
Depends On:
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Reported: 2008-04-24 14:53 EDT by John Shakshober
Modified: 2009-03-10 08:39 EDT (History)
7 users (show)

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Fixed In Version:
Doc Type: Bug Fix
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Last Closed: 2009-03-10 08:39:57 EDT
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RHEL 7.3 requirements from Atomic Host:
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Description John Shakshober 2008-04-24 14:53:12 EDT
Description of problem:
Running IOzone to an fedora RHEL5 or f9 guest, adding -I flag for directI/O give
3-4x the performance of the raw device.  

Version-Release number of selected component (if applicable):

How reproducible:


Steps to Reproduce:
1. copy iozone kit http://people.redhat.com/dshaks/.kits/iozone-3-233.i386.rpm
2. moutn a data disk in the guest (either file or phy based) 
3. ./iozone -s 1g -r 64k -I -f /perf1
Actual results:
f9_base achieved             161 MB/sec
f9_fv 	in fv guest achieved 785 MB/sec

Expected results:
<= 161 MB/sec as it can't run faster than the raw device

Additional info:
Comment 1 Mark McLoughlin 2008-04-25 03:21:52 EDT
Just to make it clear for everyone reading - the issue with the fact that FV is
outperforming baremetal is that it implies that writes aren't being flushed to
the host's disk before being acknowledged which, in turn, means the guests could
be corrupted when something goes wrong.
Comment 2 Red Hat Bugzilla 2008-05-14 07:28:28 EDT
Changing version to '9' as part of upcoming Fedora 9 GA.
More information and reason for this action is here:
Comment 3 Daniel Berrange 2008-10-07 15:18:01 EDT
Core problem here is that libvirt is not setting the 'cache=off' flag to -drive argument when launching QEMU.
Comment 4 Daniel Berrange 2008-10-08 07:07:10 EDT
Patch posted

Comment 5 Daniel Berrange 2009-03-10 08:39:57 EDT
libvirt now has the ability to set the caching mode in the XML for a disk, as of 0.6.0 release. So going to close this bug now.

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