KVM storage performance and cache settings on Red Hat Enterprise Linux 6.2

Written by Gionatan Danti on . Posted in Virtualization

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Testbed and methods

All test were run on a Dell D620 laptop. The complete system specifications are:

  • Core2 T7200 CPU @ 2.0 GHz
  • 4 GB of DDR2-667 RAM
  • Quadro NVS110 videocard
  • a Seagate ST980825AS 7200 RPM 80 GB SATA hard disk drive (in IDE compatibility mode, as the D620's BIOS does not support AHCI operation)
  • OS Red Hat Enterprise Linux 6.2 amd64

Kernel version was kernel-2.6.32-220.el6.x86_64 while Qemu/KVM version was qemu-kvm-0.12.1.2-2.209.el6_2.1.x86_64. The internal hard disk was partitioned into three slices: a first ~4 GB swap partition, a second ~20 GB ext4 root partition and a third ~50 GB ext4 partition (mounted on /opt) for testing purposes. All guest's disk images were stored on this last partition.

I installed a Debian 6 amd64 OS in a guest instance backed by a Qcow2 image file. The Debian installation process was repeated multiple times, both with EXT3 and EXT4 root filesystems. Please note that when using EXT3-based root filesystem, Debian installation proceeded without write barriers support, as the installer does not permit their activation from the default text-based menu; EXT3 write barriers were enabled after the installation was completed by setting the correct option in /etc/fstab. On the other hand, EXT4 uses write barriers by default.

Write performances were benchmarked by measuring the time needed to install the Debian base system, while I used the Linux “dd” tool to check for correct Qemy/KVM barrier-passing functionality.

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