KVM scalability and consolidation ratio: cache none vs cache writeback

Written by Gionatan Danti on . Posted in Virtualization

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Testbed and methods

Benchmarks were performed on a system equipped with:

  • PhenomII 940 CPU (4 cores @ 3.0 GHz, 1.8 GHz Northbridge and 6 MB L3 cache)
  • 8 GB DDR2-800 DRAM (in unganged mode)
  • Asus M4A78 Pro motherboard (AMD 780G + SB700 chipset)
  • 4x 500 GB hard disks (1x WD Green, 3x Seagate Barracuda) in AHCI mode, configured in software RAID10 "near" layout
  • S.O. CentOS 6.5 x64

The operation system was installed with “basic server” profile and then I selectively installed the other softwares required (libvirtd, qemu, etc). Key systems softwares were:

  • kernel-2.6.32-431.1.2.0.1.el6.x86_64
  • qemu-kvm-0.12.1.2-2.415.el6_5.3.x86_64
  • libvirt-0.10.2-29.el6_5.2.x86_64

To test KVM in a true multi-guests environment, I created a basic “tile” of four guests, each with VirtIO drivers in place (for both disk and network devices). A tile is composed by:

  • two (#1 and #2) Windows Server 2012 R2 64 bit guests, each with 1 GB RAM and 32 GB disk
  • two CentOS 6.5 x86_64 guests (#3 and #4), each with 512 MB RAM and 8 GB disk.

All virtual machines use LVM-based disks, carved out by a dedicated volume group.

Inside the tile, each VM has the following role:

  • the two (#1 and #2) Windows Server 2012 R2 64 bit guests act as fileservers. The first Win2012 guest copies, via SMB/CIFS, a ~670 MB directory (with over 44500 files) on the second Win2012 guest. After 30 seconds of idling, it copies back the directory from the peer Win2012 machine.
  • the first CentOS 6.5 x86_64 guest (#3) acts as a dynamic web and email server (using apache, mysql and postfix). This machine serves a Joomla 3.2.5 site. At the same time, it runs a postgresql benchmark (sysbench) against the fourth guest, issuing a total of 10.000 transactions with a concurrency level of 4.
  • the second CentOS 6.5 x86_64 guest (#4) acts as a pure database server (using postgresql). This guest is benchmarked by the previous CentOS VM, and at the same time it runs AB (apache benchmark) to stress it, issuing 2.000 requests with a concurrency level of 4. Moreover, it run a shell script generating 100 batches of 5 emails, each of ~56 KB, with one second wait between each batch. Finally, it sends 100 of those emails in a single, big burst. Totally, it moves about 34 MB of data.

In short, those VMs benchmark each other. This ensure not only that the test is self-contained, without externally-induced variables, but it also stress the internal virtual network switch created by Qemu.

I perfectly understand that I am testing very specific scenarios, so let me know what you, the reader, think about that. Do you want a more web-specific test case? Your focus is on database performance? Or fileserver speed is all that matter to you?

Let me know your ideas!

Comments   

 
#1 Troels Arvin 2014-01-20 20:24
Where can one read more about the potential issues with write barriers and live migration?
 
 
#2 Gionatan Danti 2014-01-21 10:20
Quoting Troels Arvin:
Where can one read more about the potential issues with write barriers and live migration?


Hi,
here you can find some informations on barriers:
http://www.ilsistemista.net/index.php/virtualization/23-kvm-storage-performance-and-cache-settings-on-red-hat-enterprise-linux-62.html?start=2

On live migration and why it is better to not use it with caching, you can read libvirt and qemu man page.

Regards.
 

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