Re: IO on guest is 20 times slower than host

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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?

--
error compiling committee.c: too many arguments to function

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