Bugs item #2926083, was opened at 2010-01-05 10:16 Message generated for change (Comment added) made by avik You can respond by visiting: https://sourceforge.net/tracker/?func=detail&atid=893831&aid=2926083&group_id=180599 Please note that this message will contain a full copy of the comment thread, including the initial issue submission, for this request, not just the latest update. Category: qemu Group: None >Status: Closed >Resolution: Wont Fix Priority: 5 Private: No Submitted By: Baka Project (bakaproject) Assigned to: Nobody/Anonymous (nobody) Summary: qcow2 default cache slows 0.12.1.2 vs kvm-88 Initial Comment: An install of a linux guest closely resembling RHEL5.4 x86_64 (Linux rats1 2.6.18-164.6.1.el5 #1 SMP Tue Oct 27 11:28:30 EDT 2009 x86_64 x86_64 x86_64 GNU/Linux) is extremely slow in 0.12.1.2 compared to kvm-88. Some extensive git bisecting revealed the problem being that the default cache strategy for qcow2 was changed from writeback to writethrough. When I run qemu in writeback mode, the performance is restored. I understand the performance/correctness tradeoff here--after wasting a day tracking it down. I guess this is a plea to restore the previous behavior or better advertise the change or potential solutions to the problem. qemu command (without explicit cache): aoss x86_64-softmmu/qemu-system-x86_64 -enable-kvm -name rats2 -hda /VMs/images/rats2.cow -m 650 -monitor telnet:localhost:9008,server,nowait,nodelay -vnc :8 -cdrom /VMs/cds/curodw.iso -boot dc -net nic,model=rtl8139,macaddr=2e:2e:2e:0:0:db,vlan=0 -net tap,ifname=rats2_0,vlan=0,script=no,downscript=no -net nic,model=rtl8139,macaddr=2e:2e:2e:0:1:db,vlan=1 -net tap,ifname=rats2_1,vlan=1,script=no,downscript=no -net nic,model=rtl8139,macaddr=2e:2e:2e:0:2:db,vlan=2 -net tap,ifname=rats2_2,vlan=2,script=no,downscript=no -serial telnet:localhost:9108,server,nowait,nodelay -serial telnet:localhost:9208,server,nowait,nodelay -daemonize -pidfile /VMs/var/pids/rats2 -usbdevice tablet Command with explicit cache: aoss x86_64-softmmu/qemu-system-x86_64 -enable-kvm -name rats2 -drive file=/VMs/images/rats2.cow,if=ide,index=0,cache=writeback -m 650 -monitor telnet:localhost:9008,server,nowait,nodelay -vnc :8 -cdrom /VMs/cds/curodw.iso -boot dc -net nic,model=rtl8139,macaddr=2e:2e:2e:0:0:db,vlan=0 -net tap,ifname=rats2_0,vlan=0,script=no,downscript=no -net nic,model=rtl8139,macaddr=2e:2e:2e:0:1:db,vlan=1 -net tap,ifname=rats2_1,vlan=1,script=no,downscript=no -net nic,model=rtl8139,macaddr=2e:2e:2e:0:2:db,vlan=2 -net tap,ifname=rats2_2,vlan=2,script=no,downscript=no -serial telnet:localhost:9108,server,nowait,nodelay -serial telnet:localhost:9208,server,nowait,nodelay -daemonize -pidfile /VMs/var/pids/rats2 -usbdevice tablet Kernel modules: 2.6.32.2ish (gentoo-sources 2.6.32-gentoo-r1) configure: --audio-drv-list=alsa --kerneldir=/lib/modules/2.6.32-gentoo-r1/build CPU: Intel(R) Core(TM)2 Extreme CPU X9650 @ 3.00GHz (family 6, model 23, stepping 6, 6MB cache, 4 cores) ---------------------------------------------------------------------- >Comment By: Avi Kivity (avik) Date: 2010-01-05 10:58 Message: The default must of course be safe. Is there some reason why you can't use cache=writeback if you don't care about data integrity? Note performance with old images (created with 4K cluster size) will be much lower than new images (created with 64K cluster size). Check your image cluster size. ---------------------------------------------------------------------- You can respond by visiting: https://sourceforge.net/tracker/?func=detail&atid=893831&aid=2926083&group_id=180599 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
