Virtualization Blog

Discussions and observations on virtualization.
Andrew Halley 1st encountered virtual systems at CERN before going onto to be a heavy user of globally distributed Linux systems for research analysis. Moving from academia to industry he then developed software for mobile protocol stacks and applications frameworks before switching to running global projects and software teams. These have ranged from cellular, and GPS to secure communications for emergency services and now (full-circle!) into the work of virtualised IT and Cloud infrastructure with Citrix XenServer Development.

Making a difference to the next release

With the 1st alpha build of the new, upcoming XenServer release (codenamed " Ely") as announced by Andy Melmed's blog on the 11th October – I thought it'd be useful to provide a little retrospective on how you, the community, have helped getting it off to a great start by providing great feedback on previous alphas, betas and releases - and how this has been used to strengthen the codebase for Ely as a whole based on your experiences.
As I am sure you are well aware - community users of can make use of the incident tracking database at to raise issues on recent alphas, betas and XenServer releases to raise issues or problems they've found on their hardware or configuration.  These incidents are raised in the form of XSO tickets which can then be commented upon by other members of the community and folks who work on the product.  
We listened
Looking back on all of the XSO tickets raised on the latest 7.0 release - these total more than 200 individual incident reports.  I want to take the time to thank everyone who contributed to these, often detailed, specific, constructive reports, and for working iteratively to understand more of the underlying issues.  Some of these investigations are ongoing, and need further feedback, but many of them are sufficiently clear to move forward to the next step.  
We understood
The incident reports were triaged and, by working with the user community, more than 80% of them have been processed.  Frequently this involved questions and answers to get a better handle on what was the underlying problem.  Then trying a change to the configuration or even a private fix to see and confirm if it related to the problem or resolved it.  The enthusiasm and skill of the reporters has been amazing, and continually useful.  At this point - we've separated the incidents into those which can be fixed as bugs, and those which are requests for features.  The latter have been provided to Citrix product management for consideration.  
We did
Out of these which can be fixed as bugs,  we raised or updated 45 acknowledged defects in XenServer.  More than 70% of these are already fixed - with another 20% being actively worked on.  The small remainder are blocked for some reason and awaiting a change elsewhere in the product, upstream or in our ability to test.  The 70% of fixes have now successfully either become part of some of the hotfixes which have been released for 7.0, or are in the codebase already and are being progressively released as part of the Ely alpha programme for the community to try.  
So what's next?  With work continuing apace on Ely - we have now opened the "Ely alpha" as a affects-version in the incident database to raise issues with this latest build.  At the same time - in the spirit of continuing to progressively improve the actively developing codebase - we have removed the 6.5 SP1 affects-version – so folks can focus on the new release.
Finally - on behalf of all users - my personal thanks to everyone who has helped improve both Dundee and Ely - both by reporting incidents, triaging and fixing them and by continuing to give your feedback on the latest new version.  This really makes a difference to all members of the community.
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Behind the Mountain - Load Testing XenServer 6.5 SP1

Behind the Mountain - Load Testing XenServer 6.5 SP1

With the right equipment (CPU and memory) – previous releases of XenServer like 6.1 are able to run up to 150 concurrent instances of Microsoft Windows and Linux VMs on a single server. More recent releases like 6.2 and 6.5 have pushed this envelope to the higher and higher altitudes of 500 and 650 Windows and Linux VMs respectively. The newly released 6.5 SP1 version has climbed yet higher into the stratosphere and is now able to run an amazing 1000 Windows 7 VMs. The next question is always - OK so the system can run these large numbers of VMs - but how many are useful in real-world environments and use-cases?

To answer this - we carry out detailed performance investigations to understand how well the system behaves under LoginVSI loads which simulate a typical knowledge-worker as a desktop (email, browser, apps etc.). In the case of the following test, the workload was a LoginVSI Medium one. We then determine the maximum number of such VMs which can be run like this, whilst offering the end-user really good performance and responsiveness.  

  • Starting with changes made to XenServer 6.5 - we show here that XS 6.5 can handle 500 VMs with every single one of them performing acceptably. Thus XenServer 6.5 enjoys a massive 40% improvement over XenServer 6.2 in this metric, and a massive 125% improvement over XenServer 6.1. Full details and plots etc. are shown in the article here.
  • On top of this - we have now completed the same measurements using XenServer 6.5 SP1. From this, I am very happy to say that 6.5 SP1 is able to run yet more, a lot more. In fact - an astonishing 20% more responsive VMs than 6.5 using the LoginVSI 3.5 workload. It gives a LoginVSI max score of 600 out of its maximum 1000 Windows 7 VMs which we can run on this host.  

With a bigger server, we fully expect XenServer to be able to run 1000 with the same responsiveness for full desktop workloads like LoginVSI. The full details and plots etc. are shown in the article here.

Recent Comments
Tobias Kreidl
Thanks for the post, Andy. It would be useful to have metrics for Windows 8.X and, come to think of it, also Windows 10 VMs once a... Read More
Thursday, 13 August 2015 17:34
Andrew Halley
Thanks Tobias. We're in the process of upgrading our LoginVSI suite to 4.0. The newer versions of LoginVSI have more intensive w... Read More
Thursday, 03 September 2015 15:56
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Advisory for users of Space Reclamation (TRIM) in XenServer 6.5

We've had an important advisory for users who are trying the new XenServer 6.5 release and would like to use the new Space Reclamation feature (TRIM) on LUNs bigger than 2 TiB  as we have found an issue (inherited from upstream) in the original release which might result in data corruption of virtual disks under certain conditions such as high I/O. 

This issue has now been fixed so please apply Hotfix XS65E005 as soon as possible which has been released to address this issue.  More information is available here Http://

In XenCenter, this functionality is labeled "Reclaim Freed Space" under the host's storage tab.

Note - this doesn't affect other functionality in the product - and is now resolved by applying this hotfix.

Recent Comments
JK Benedict
Thanks, Andy!
Friday, 06 February 2015 19:08
Andrew Halley
Thanks Thilko. Understood. A fix is under test. Will let you know as soon as I know of an ETA for a hotfix.
Monday, 09 February 2015 09:12
Andrew Halley
Sorry if I'm not understanding the question correctly. XenServer 6.5 is very much safe to use. This issue only affects users us... Read More
Wednesday, 11 February 2015 16:16
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XenServer Pre-Release Programme

A very big thank you for everyone who participated in the Creedence Alpha/Beta programme! 
The programme was very successful and raised a total of 177 issues, of which 138 were resolved during the Alpha/Beta period.  We are reviewing how the pre-release process can be improved and streamlined going forward. 

The Creedence Alpha/Beta programme has now come to an end with the focus of nightly snapshots moving on to the next version of XenServer.   

The Creedence Alpha/Beta source code remains available and can be accessed here:

Creedence Alpha/Beta bugs may still be reported on

Work is already progressing on the next version of XenServer and the nightly snapshots are available here:

As this work is new and still expected to be unstable, please do not raise any Creedence Alpha/Beta bugs against it.

Recent Comments
Andrew Halley
Hi there, we are working towards posting an updated build containing all the bug fixes received to date, and which is fully integr... Read More
Sunday, 14 December 2014 12:33
Tobias Kreidl
Andrew, Thanks go to you and the whole Citrix team for making this a really great overall experience. Each XenServer release seems... Read More
Wednesday, 10 December 2014 19:45
Andrew Halley
Appreciate it Tobias - and our thanks to all the excellent contributions received from our community.
Sunday, 14 December 2014 12:33
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About XenServer

XenServer is the leading open source virtualization platform, powered by the Xen Project hypervisor and the XAPI toolstack. It is used in the world's largest clouds and enterprises.
Technical support for XenServer is available from Citrix.