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Old 08-22-2011, 11:33 PM   #1
dman777
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Raw with Write Passthrough Cache disabled on sata for best perfomance?


I am going to run a small Qemu KVM linux guest(10 gigs) on my linux host...all running on my ssd drive. The guest will never grow in size so I was thinking about using raw instead of qcow2 for best performance. I read with raw it is best performance wise to disable write pass through cache. But this test was done on a IDE. I am using sata with AHCI divers. Would this still hold true?
 
Old 08-23-2011, 12:33 AM   #2
macemoneta
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I don't think you will notice any performance difference between preallocated qcow2 and raw. However, virtio will make a significant difference. For example:
Code:
/usr/bin/qemu-kvm -m 2048 -smp 2 -vga vmware \
                  -drive file=Disk.img,if=virtio,index=0,media=disk,boot=on \
                  -net nic,model=virtio,macaddr=52:54:00:12:34:56 -net user
 
Old 08-23-2011, 05:20 AM   #3
dman777
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http://www.ilsistemista.net/index.ph...6.html?start=5 shows that with only WT enabled on qcow2 it improved performance. But with WT disabled the perfomance was better without VirtIO. What is your opinion on that?

Last edited by dman777; 08-23-2011 at 05:34 AM.
 
Old 08-23-2011, 10:12 AM   #4
macemoneta
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Lower bars are better in that chart, and virtio performance is also a function of the kernel (generally, newer=better).
 
Old 08-23-2011, 02:03 PM   #5
dman777
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ya, but by that chart only when WT was enabled virtio offered an improvement. without WT, virtio did not help at all.
 
Old 08-23-2011, 02:22 PM   #6
macemoneta
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Unless you are planning on running Windows Server 2008 R2 Foundation under RHEL 6 that chart does not apply to your workload, but even there a slight performance improvement occurred. Virtualization performance varies with each host/guest combination, as well as with each host kernel/guest driver.
 
  


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