Re: I/O performance of VirtIO

["René Pfeiffer" <lynx@luchs.at>]

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On Oct 13, 2009 at 0145 +0400, Michael Tokarev appeared and said:
> René Pfeiffer wrote:
> >Hello!
> >
> >I just tested qemu-kvm-0.11.0 with the KVM module of kernel 2.6.31.1. I
> >noticed that the I/O performance of an unattended stock Debian Lenny
> >install dropped somehow. The test machines ran with kvm-88 and 2.6.30.x
> >before. The difference is very noticeable (went from about 5 minutes up
> >to 15-25 minutes). The two test machines have different CPUs (one is an
> >Intel Core2 CPU, the other runs with an AMD Athlon 64 X2 Dual).
> >
> >Is this the effect of added code regarding caching/data integrity to the
> >VirtIO block layer or somewhere else? The qemu-system-x86_64 seems to
> >hang a lot more in heavy I/O (showing 'D' in top/htop).
> >
> >The command line is quite straight-forward:
> >qemu-system-x86_64 -drive file=debian.qcow2,if=virtio,boot=on -cdrom \
> >/srv/isos/debian-502-i386-netinst.iso -smp 2 -boot d -m 512 -net nic \
> >-net user -usb
>   ^^^^^^^^^
> 
> Care to try with something more real than user-level networking?

Yes, I tried that on the other machine. It made not much difference (the
installation files are on local Squid proxies).

But I found that setting cache=writeback restores the old behaviour. I
think the default changed to cache=writethrough.

> You're using netinstall which - apparently - tries to use some
> networking d/loading components etc, and userlevel networking is
> known to be very very slow....

Right, I just verified the disk I/O performance with severall runs of
hdparm and fresh installations using cache=none, cache=writeback and
cache=writethrough settings. The network settings were the same (the
test machine with the software bridge setup is down at the moment). I
wanted to compare the behaviour of the I/O load.

Best,
René.

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