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Posts Tagged ‘vsphere’

I ran into an issue with a customer today where a VM was performing terribly.  From within the guest OS (a Windows 2003 application server running .NET in IIS which I will call BigBadServer) things appeared sluggish and CPU time was high.  The amount of time being spent on the kernel was notably high.  The VM in question had 4 vCPU’s and a good helping of memory.

I don’t have access to the VMware client at this particular site – just some of the guests, so I was flying bling.  Gut feeling told me that I was dealing with a resource contention issue.  I had the VMstats provider running in the guest (http://vpivot.com/2009/09/17/using-perfmon-for-accurate-esx-performance-counters/) showed me that there was no ballooning or swapping going on, and that the vCPU’s were not limited and the CPU share value seemed to be at the default.

I strongly suspected that the physical server running VMware ESX was oversubscribed on physical CPU (pCPU) resources.  Essentially, the guest VM’s that are sharing the resources of the physical machine are demanding more resources than the machine can handle.  To verify this theory, I had the client check the ‘CPU Ready’ metric on BigBadServer and bingo!

CPU Ready is a measure of the amount of time that the guest VM is ready to run against the pCPU, but the VMware CPU Scheduler cannot find time to run the VM because other VM’s are competing for the same resources.

From the stats the customer provided on our phone call, the CPU Ready for any one of the 4 vCPU’s on the BigBadServer was on average 3723ms (min: 1269ms, max:8491ms).  (Update 8/25/2010 to clarify summation stat) The summation for the entire VM was around 12,000ms on average and peaked around 35,000.  The stats came from the real-time performance  graph/table in the vSphere client. The real-time stats in the vSphere Client update every 20 seconds, so the CPU Ready summation value should be divided by 20,000 to get a percentage of CPU ready for the 20 second time slice.  If I take the worst case scenario of 8491ms per vCPU, this VM spent nearly 43% (8491/20,000) of the 20 second time slice waiting for CPU resources.

The CPU Ready summation in milliseconds counter in the vCenter Client is not always the most accurate or easy to interpret stat – to better quantify the problem it might be best to go to the ESX command line and run ESXTOP.  CPU Ready over 5% could be a sign of trouble, over 10% and there is a problem.  Running ESXTOP in batch mode and then analyzing the output using Windows Perfmon or Excel might be a good way to go on this to get a view over several hours rather than the realtime stats we were looking at.  I wrote a post a while back with more info on ESXTOP batch mode: http://vmtoday.com/2009/09/esxtop-batch-mode-windows-perfmon/

To help quantify the problem a bit more, the BigBadServer is on an ESX 4.0 server with about 10 other servers.  The physical blade has two dual-core CPU’s (AMD Opteron 2218HE’s which are not hyperthreaded).  The other VM’s on the blade have different vCPU and vMemory configurations.  3 VM’s (including BigBadServer) have 4 vCPU’s.  A couple have 2 vCPU’s, and the remainder are configured with 1 vCPU.  In ESX 4.x, the VMware console OS actually runs as a hidden VM, pegged to pCPU #1.

I generally recommend a pCPU:vCPU ration of 1:4 for mid-sized VMware deployments of single vCPU VM’s.  The blade we are running on is a 1:5 with several multi-vCPU VM’s.  The multi-vCPU’s start to skew the ratio recommendation and require some advanced design decisions.  VMware’s scheduler requires that all the vCPU’s on a VM run concurrently (even if the Guest OS is trying to execute a single thread).  Also, the VMware CPU Scheduler prefers to have all the vCPU’s from a VM run on the same pCPU.  As workloads are bounced around between pCPU’s, the benefits of CPU cache are lost.  This is one of those ‘more-is-less’ situations that you run into on virtualized environments.

What this CPU Scheduler nonsense means in this case is that the 4 vCPU’s on BigBadServer have to wait until all logical pCPU’s on the box are idle (including the one that runs ESX itself) before it can run.  If ESX can’t accomplish that (we are experiencing resource contention) it starts prioritizing workloads according to what it can best run.  It is much easier to schedule the smaller VM’s, so it tends to run those on pCPU more frequently.  The larger VM’s tend to suffer a bit more than the smaller ones.  We are competing with 2 other VM’s with 4 vCPU’s that use up all of the logical pCPU’s when they need to run, as well as with the smaller VM’s.

I suggested a few ways to fix this issue for the BigBadServer web server:

1. Using Shares and/or Reservations on the VM.  This probably won’t work in our situation as the physical server is too over-subscribed.  We might see a slight improvement in BigBadServer (or we might not see any change), but possibly at the extreme expense of the other VM’s sharing the blade.
2. Reduce the number of vCPU’s on BigBadServer AND the other multi-vCPU VM’s on the same physical server.  This would reduce resource contention and open up a whole bunch of scheduling options for the VMware CPU Scheduler.  This is the quickest/cheapest fix, but will not work if the VM’s really do need 4 vCPU’s.  A little workload analysis should determine which can be made smaller (the vCenter server graphs/stats should be enough for this).  For what it’s worth, by our analysis BigBadServer seems to be happier with 4 vCPU assuming we can run with a low CPU Ready on those 4.
3. Move the BigBadServer VM to a physical ESX server with fewer multi-vCPU VM’s so there is less contention.
4. Move the BigBadServer VM to a physical ESX server with quad-core pCPU’s (ideally two quad-cores or bigger).  This would give a lot more flexibility to the VMware CPU Scheduler and allow it to run quad-vCPU VM’s on the same pCPU for greater efficiency.
5. Split BigBadServer into 2 smaller VM’s – The server currently runs a couple sites.  We could split them onto two servers – one for Project1 and one for Proejct2.  This configuration would take some design, testing, and time but could scale out better, give more flexibility and availability in the long run.

I’m not sure which way the customer will go on this one yet, but I feel good having armed them with enough knowledge and options to make an informed decision.

To avoid problems like this in the future, I recommend these rules of thumb:

• Design your hosts for your guests.  Taking your Guest VM sizes into account when designing your environment and choosing physical hardware is crucial if you need bigger VM’s.
• Don’t make your VM’s bigger than you have to.  It is always easier to add resources than take them away.  Hot Add of CPU and Memory in vSphere make adding incredibly easy.
• Monitor your environment for CPU Ready, Swapping, and other metrics that can indicate an inefficient design.
• Call for help when you can’t figure out what is going on (I’m happy to help!).  VMware is super powerful, but some things can be downright backwards when it comes to resource allocation on a fixed set of hardware.

If you are looking for some resources to help explain CPU Scheduling a bit more, I recommend:

• VMware’s Official documentation of CPU Scheduler in vSphere 4.1 – http://www.vmware.com/files/pdf/techpaper/VMW_vSphere41_cpu_schedule_ESX.pdf.
• A nice summary of co-scheduling from VMware’s Performance Blog: http://blogs.vmware.com/performance/2008/06/esx-scheduler-s.html
• Description and stats on Ready Time metrics for VI3: http://www.vmware.com/pdf/esx3_ready_time.pdf
• Understanding Virtual Center Performance Statistics: http://communities.vmware.com/docs/DOC-5230.pdf

(Updated 8/25/2010 to include a few additional reference links and corrected summation divided by time slice to get accurate values)

I upgraded my in-house VMware vSphere environment to 4.0 Update 2 last week.  After upgrading my vSphere Client to the Update 2 version I was greeted with a series of 7 ugly error messages stating: Method not found: ‘VMware.CustomControls.LabelEx VpxClient.Common.Util.Helper.AddLabel(System.Windows.Froms.Control,Int32, int32, System.String, System.Drawing.FontStyle, Boolean)”.I assumed a plug-in had caused the error message.  I started my troubleshooting by disabling the 3rd-party plug-ins in the environment, beginning with the free EMC Storage Viewer.  Upon disabling the EMC Storage Viewer 2.x plug-in the problem was resolved.  I went out to EMC PowerLink to see if an update was available for the plug-in and was surprised to find that I had missed a major update/rebranding of the plug-in.  EMC now calls the plug-in the ‘EMC Virtual Storage Integrator’.  A hotfix was released on July 2nd to bring Update 2 support to the plug-in (version 3.0.0.32).

I updated the Solution Enabler installation (I installed Solution Enabler on my vCenter server, but it is also available as a SUSE based virtual appliance), and then updated the plug-in.

The update appeared to install without any problems.  The vSphere Client launched like a champ after the update – no errors, but no EMC Storage plug-in either.  Odd.  The problem occurred on both my vCenter Server’s vSphere client and my workstation, so seems to be more than an isolated issue.

I did an uninstall of the plug-in using Add/Remove Programs, and then reinstalled it.  After the reinstall the EMC Storage plug-in icon appeared in my vSphere Client as pictured below.

The EMC Virtual Storage Integrator (VSI) plug-in is free – installing it is a no-brainer for anyone running Clariion or Celerra storage arrays.  The VSI simplifies the job of mapping vSphere Datastores to LUN’s and NFS shares on your EMC storage, and helps pinpoint the location of VM’s and RDM’s on your array.  This visibility for the VMware administrator into the storage layer can go a long way in helping troubleshoot storage performance issues and simplifying communication between server, storage, and virtulization teams.

EMC actually offers three different free vSphere plug-ins, including the VSI.  The EMC Unified Block plug-in and the EMC Unified NAS plug-in round out the trio.  EMC’s Virtual Geek, Chad Sakac, covers all three in his blog post here: http://virtualgeek.typepad.com/virtual_geek/2010/06/update-on-emc-vsphere-plugins.html.

I am finishing up an installation of an EMC Clariion CX4 SAN. One of the final steps of the installation is to configure PowerPath/VE on the ESXi hosts. PowerPath/VE is EMC’s multipathing extension module for VMware (and Hyper-V), designed to replace the Native Multipathing Plugin (NMP) for increased I/O performance and failover management.  To simplify and automate the installation of PowerPath/VE, I decided to use VMware Update Manager (VUM) to push the extension to the ESXi 4.x hosts in the environment.

The process of setting up an additional VUM patch repository to host PowerPath/VE (and other 3rd party extensions such as the Cisco Nexus 1000v) is pretty straight forward.  3rd party extensions are supported in VUM beginning with vSphere 4.0 Update 1.  Chad Sakac has posted a great video guide on YouTube that covers the setup:

I opted to use the tomcat installation on the environment’s vCenter server to host the PowerPath/VE repository.  To accomplish this, I simply created a new directory in the tomcat root directory.  The default path for the root directory on a vSphere vCenter Server is “C:\Program Files\VMware\Infrastructure\tomcat\webapps” (or C:\Program Files (x86)\VMware\Infrastructure\tomcat\webapps on a 64-bit installation).

I created a directory named ‘depot’ and within that directory created a PowerPathVE folder.  I extracted the contents of the VUM folder from the PowerPath .zip file that I downloaded from http://powerlink.emc.com.  A screenshot of the directory is below:

After creating the directory for the patch repository, I simply added an Extension Repository to VMware Update Manager as Chad shows in his video.  I would like to call out one caveat – Because vCenter may not listen on standard HTTP/HTTPS ports, I used https://vcenter.domain.local:8443/depot/PowerPathVE/index.xml as the path to the source.

Once PowerPath was added to an Extension Baseline in VUM, I simply had to scan my hosts for updates and remediate.  Installation of PowerPath/VE requires the host to be in Maintenance Mode and concludes with a reboot.  Pretty simple.

Then all you have to do is fight through an overly-complex licensing setup (seriously, a 112 page PDF on how to install licenses???), a bit of configuration, and you are multi-pathing with the best of them.  If you are interested in learning more about PowerPath/VE, start with this whitepaper: EMC PowerPath/VE for VMware vSphere Best Practices Planning.  For a bit of real-world insight into the performance increase you might see with PowerPath/VE, check out this blog post from Eric Sloof: Massive I/O power increase using EMC PowerPath/VE.

Update – 3/27/09: VMware published a Knowledge Base article on this procedure a few weeks after I wrote this post.  You can find it in  article 1018740.

I recently posted an article on how specific actions during the upgrade of a VMware Virtual Machine’s hardware from v4 to v7 can cause problems with certain services, including DNS, DHCP, and WINS. In that case, the problem was related to Microsoft Windows leaving non-present devices with networking configurations and  the failure of the VMware Upgrade Helper service to copy WINS settings when updating the NIC.  As my fellow blogger and VMUG leader, Jason Boche, responded on Twitter: “Same gotchas, different version.”  And right he is – anyone with experience in P2V or V2V, or who has been working with VMware long enough to have done a 2.5 to 3.0 upgrade experienced the same gotchas.

There are other issues with VMware virtual hardware upgrades, however, that you may not have experienced.  One such issue that I have experienced is highlighted in VMware Knowledge Base article 1013109: “Upgrading virtual hardware in ESX 4 may cause Windows 2008 disks to go offline“.  The problems described in the article are unique to Windows 2008 Enterprise and Datacenter editions only.  The problem is pretty well described in the title of the article – Upgrading virtual hardware in ESX 4 may cause Windows 2008 disks to go offline.  In this case, like with the ghost NIC’s I described last week, is more of a Microsoft issue, but it will rear its head when a VMware Administrator least desires it.  With this particular problem, the Windows Virtual Disk Service (part of the native Storage Management suite) is set to not auto-mount newly discovered disks that do reside on a shared bus.  Microsoft has a MSDN article on the VDS SANS policy here.  Upgrading the virtual hardware version causes the disks to be re-discovered and not auto-mounted.  This can potentially impact all non-system disks on a VM.

You may also experience similar issues when upgrading the vSCSI adapter in a VM from a standard LSI Logic Parallel SCSI adapter to a (new in vSphere 4.0) paravirtualized SCSI (pvSCSI) adapter, move virtual disks to new vSCSI adapters to increase the number of concurrent disk IO operations, or when you change the SCSI node ID of a virtual disk.  These may all trigger a re-discovery of the disks by the Windows Virtual Disk Service, leaving data disks offline on Windows 2008 Enterprise and Datacenter Edition guests.

In my opinion, these issues are not reasons to forgo upgrading your virtual hardware version.  However, when your upgrade/migration plans call for upgrading the virtual hardware version of your guests you should be prepared to resolve any issues caused by ‘ghost hardware’, offline disks, and the like.  Both the MSDN and VMware articles I cited above offer workarounds for the offline disk issue.  Here are the links again:

http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1013109

http://msdn.microsoft.com/en-us/library/bb525577%28VS.85%29.aspx