Re: IO on guest is 20 times slower than host

[Avi Kivity <avi@redhat.com>]

Kurt Yoder wrote:
> Hello list,
>
> I'm puzzled as to why my KVM guest has IO that is 20 times slower than 
> the KVM host:
>
> me@guest:~$ dd if=/dev/zero of=/tmp/bigfile count=100000
> 100000+0 records in
> 100000+0 records out
> 51200000 bytes (51 MB) copied, 8.36612 s, 6.1 MB/s
> me@guest:~$ df -h
> Filesystem            Size  Used Avail Use% Mounted on
> /dev/vda1             9.2G  587M  8.2G   7% /
> tmpfs                 123M     0  123M   0% /lib/init/rw
> varrun                123M   28K  123M   1% /var/run
> varlock               123M     0  123M   0% /var/lock
> udev                  123M  2.6M  120M   3% /dev
> tmpfs                 123M     0  123M   0% /dev/shm
>
>
>
> me@host:~$ dd if=/dev/zero of=/tmp/bigfile count=100000
> 100000+0 records in
> 100000+0 records out
> 51200000 bytes (51 MB) copied, 0.47188 s, 109 MB/s
>
>
>
> The VM is started via libvirt and is the only VM running on an 
> otherwise-unused system:
>
> root     19915     1  0 Mar24 ?        00:00:00   /usr/sbin/libvirtd -d
> nobody   19937 19915  0 Mar24 ?        00:00:00     dnsmasq 
> --keep-in-foreground --strict-order --bind-interfaces --pid-file  
> --conf-file  --listen-address 192.168.122.1 --except-interface lo 
> --dhcp-leasefile=/var/lib/libvirt/dhcp-default.leases --dhcp-range 
> 192.168.122.2,192.168.122.254
> root     24354 19915  2 09:52 ?        00:02:33     /usr/bin/kvm -S -M 
> pc -m 256 -smp 1 -name guest -monitor pty -boot c -drive 
> file=/dev/HW_RAID/Guest,if=virtio,index=0,boot=on -net 
> nic,macaddr=52:54:00:ba:e6:db,vlan=0,model=virtio -net 
> tap,fd=11,script=,vlan=0,ifname=vnet1 -serial none -parallel none -usb 
> -vnc 127.0.0.1:1
>
>
>
> Can someone suggest where to start looking? Thanks,

I get 141 MB/s on the same test (it only copies to memory anyway).  Is 
your VM under memory pressure?

-- 
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