[GLLUG] KVM Performance

John Hearns hearnsj at gmail.com
Wed Jun 10 19:20:13 UTC 2020


On Tue, 9 Jun 2020 at 21:52, Ken Smith via GLLUG <gllug at mailman.lug.org.uk>

> Hi All,
> While in lockdown I decided to do some performance testing on KVM. I had
> believed that passing a block device through to a guest rather than
> using a QCOW2 file would get better performance. I wanted to see whether
> that was true and indeed whether using iSCSI storage was any better/worse.
> My test hardware is quite modest and this may adversely have affected
> what I measured. The processor is a Intel Core2 6300  @ 1.86GHz with
> VT-X support. It shows 3733 Bogomips at startup. There's 8GB RAM and an
> Intel 82801HB SATA controller on a Gigabyte MB. The disks are two 3TB
> SATA 7200RPM set up with a Raid 1 LVM Ext3 partition as well as other
> non-Raid partitions to use to test.
> I used Fedora 32 as the KVM host and my testing was with Centos 8 as a
> guest.
> On the host I got 60MB/s write and 143 MB/s read on Raid1/LVM/Ext3. I
> wrote/read 10GB files using dd. 10Gb so as to overflow any memory based
> caching. Without LVM that changed to 80 MB/s write and 149 MB/s read.
> I tried all kinds of VM setups. Normal QCOW2, pass though of block
> devices Raid/LVM and Non-Raid/LVM. I consistently got around 14.5 MB/s
> write and 16.5 MB/s read. Similar figures with iSCSI operating from both
> file based devices and block devices on the same host. The best I got by
> tweaking the performance settings in KVM was a modest improvement to 15
> MB/s write and 17 MB/s read.
> As a reference point I did a test on a configuration that has Centos 6
> on Hyper-V on an HP ML350 with SATA 7200 rpm disks. I appreciate that's
> much more capable hardware, although SATA rather than SAS, but I
> measured 176 MB/s write and 331 MB/s read. That system is using a file
> on the underlying NTFS file system to provide a block device to the
> Centos 6 VM.
> I also tried booting the C8 guest via iSCSI on a Centos6 Laptop, which
> worked fine on a 1G network. I measured 16.8 MB/s write and 23.1 MB/s
> read that way.
> I noticed an increase in processor load while running my DD tests,
> although I didn't take any actual measurements.
> What to conclude? Is the hardware just not fast enough? Are newer
> processors better at abstracting the VM guests with less performance
> impact? What am I missing??
> Any thoughts from virtualisation experts here most welcome.
> Thanks
> Ken
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