PVHVM CentOS 7 on XenServer

In this post:

  1. Benchmarks
  2. Prebuilt image
  3. Kickstart script

Following my previous post on running CentOS 7 and Ubuntu 14.04 as fully-paravirtualized guests on XenServer, I ran some benchmarks to compare the relative performance of fully-paravirtualized (henceforth abbreviated PV) domUs against HVM guests using paravirt drivers and interrupts/timers (henceforth PVHVM).

The performance differences between the two types has been studied for some time. Once upon a time, PV was undoubtedly faster, free of the overhead associated with full hardware emulation. With newer hardware features that have been supported for the last few years, PVHVM, which takes advantage of features in the processor as well as a Linux kernel that recognizes that it is operating as a virtual guest, has surpassed PV performance in most arenas.

Benchmarks

Benchmarks have severe limitations. The statistics here are only meant to be compared relatively among themselves—between the PV and PVHVM guests running exactly the same specs and software. It would be a futile exercise to compare them against VMs running on other servers, which might have better SANs, lighter workloads, or faster CPUs and RAM. The specific test profiles in the Phoronix software are also based on outdated versions of Apache httpd and nginx, which makes them unreliable for assessing real-world performance.

Some of the relevant comparisons:

It’s worth noting that CentOS 7 with a 3.10 kernel performed poorly compared to other distributions—both Fedora 20 (kernel 3.15) and Ubuntu 14.04 (kernel 3.13) outperformed CentOS on web serving workloads (not shown). But the evidence pretty conclusively showed that PVHVM generally performed better than PV on all of the operating systems.

Prebuilt image

Update (2017-04-28): Because these images are now out of date and insecure, the .xva images have been deleted. You should instead use the distribution’s latest cloud images in .qcow2 format, converted for XenServer.

To that end, I’d like to offer a prebuilt CentOS 7.0.1406 image that runs in PVHVM on XenServer. You should feel free to choose between this and the PV version from my previous post, depending on your needs. (If you need to accommodate higher density on your server, you might be better off with PV. Run benchmarks yourself to decide what you should use.)

As before, you can decompress (xz -d ___.xvz.xz or use your GUI of choice) then import through XenCenter (File – Import…) or the command line (xe vm-import filename=___.xva).

This image is provided with no guarantees. Please let me know in the comments if you find an issue with it.

  • CentOS 7.0.1406 (as of 2014-07-31)
    Filename: centos-7.0.1406-20140731-pvhvm-template.xva.xz
    Size: 325 MB xz-compressed; 1.4 GB decompressed
    Specs: 2 vCPUs, 2 GB RAM, 8 GB disk without swap, installed software, with XenServer Tools 6.4.93
    SHA256 hash: c3ef221ae886cea4c3be09996d0cb2049dc2ac8f10dd5323f85beee25ce9d4cd
    MD5 hash: 44583aa3cdbf1db1c62b2db05530ce6f
    Username: centos
    Password: Asdfqwerty

Kickstart script

A PVHVM system requires no special accommodations when installing, except that UEFI and GPT are not certain to be supported. Merely select the “Other install media” option in XenCenter, and use a standard installer ISO/DVD. Do NOT use any of the CentOS or RHEL templates! Those will create PV guests.

An automated kickstart like the one used to create the image above may help you build a generic template. Hit <Tab> at the CentOS DVD menu and append a ks=__ parameter to use a kickstart script hosted at an HTTP location.

The image above was built with the cent70-server-pvhvm.ks script at revision e278f2a8139fb624bc2cdcd9a80d8b51b7910de3, embedded below. If there are any updates to this script, they will be added to the develop branch on GitHub. You can also edit it yourself before deploying.

Did this help you?

If you were able to use this image or the kickstart, I’d appreciate a brief comment to let me know it worked for you. I’d hope that the bandwidth costs are going to good use!