Yellow Bricks

by Duncan Epping

5.0

Permanent Device Loss (PDL) enhancements in vSphere 5.0 Update 1 for Stretched Clusters

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In the just released vSphere 5.0 Update 1 some welcome enhancements were added around vSphere HA and how a Permanent Device Loss (PDL) condition is handled. The PDL condition is a condition that is communicated by the array to ESXi via a SCSI sense code and indicates that a device (LUN) is unavailable and more than likely permanently unavailable. This is a condition which is useful for “stretched storage cluster” configurations where in the case of a failure in Datacenter-A the configuration in Datacenter-B can take over. An example of when a condition like this would be communicated by the array would be when a LUN “detached” in a site isolation. PDL is probably most common in non-uniform stretched solutions like EMC VPLEX. With VPLEX site affinity is defined per LUN. If your VM resides in Datacenter-A while the LUN it is stored on has affinity to Datacenter-B in case of failure this VM could lose access to the LUN. These enhancements will ensure the VM is killed and restarted on the other side.

Please note that action will only be taken when a PDL sense code is issued. When your storage completely fails for instance it is impossible to reach the PDL condition as there is no communication possible anymore from the array to the ESXi host and the state will be identified by the ESXi host as an All Paths Down (APD) condition. APD is a more common scenario in most environments. If you are testing these enhancements please check the log files to validate which problem has been identified.

With vSphere 5.0 and prior HA did not have a response in a PDL condition, meaning that when a virtual machine was residing on a datastore which had a PDL condition the virtual machine would just sit there. This virtual machine would be unable to read or write from disk however. As of vSphere 5.0 Update 1 a new mechanism has been introduced which allows vSphere HA to take action when a datastore has reached a PDL state. Two advanced settings make this possible. The first setting is configured on a host level and is “disk.terminateVMOnPDLDefault”. This setting can be configured in /etc/vmware/settings and should be set to “True”. This setting ensures that a virtual machine is killed when the datastore it resides on is in a PDL state. The virtual machine is killed as soon as it initiates disk I/O on a datastore which is in a PDL condition and all of the virtual machine files reside on this datastore. Note that if a virtual machine does not initiate any I/O it will not be killed!

The second setting is a vSphere HA advanced setting called das.maskCleanShutdownEnabled. This setting is also not enabled by default and it will need to be set to “True”. This settings allows HA to trigger a restart response for a virtual machine which has been killed automatically due to a PDL condition. This setting allows HA to differentiate between a virtual machine which was killed due to the PDL state or a virtual machine which has been powered off by an administrator.

As soon as “disaster strikes” and the PDL sense code is sent you will see the following popping up in the vmkernel.log that indicates the PDL condition and the kill of the VM:

2012-03-14T13:39:25.085Z cpu7:4499)WARNING: VSCSI: 4055: handle 8198(vscsi4:0):opened by wid 4499 (vmm0:fri-iscsi-02) has Permanent Device Loss. Killing world group leader 4491
2012-03-14T13:39:25.085Z cpu7:4499)WARNING: World: vm 4491: 3173: VMMWorld group leader = 4499, members = 1

As mentioned earlier, this is a welcome enhancement which especially in non-uniform stretched storage environment can help in specific failure scenarios.

vSphere 5.0 Update 1 released

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Although only a minor version I do feel that it is worth mentioning and notifying people about. vSphere 5.0 Update 1 (click here for vCenter release notes and here for ESXi release notes) contains some cool enhancements. I listed the fixes or new features which I personally ran in to the last couple of months or which are worth implementing or important to list in specific scenarios. Especially the HA (FDM) fixes are welcome, but also the “disk.terminateVMonPDLDefault” enhancement was. I will write some more about that later today though.

vCenter Server 5.0 Update 1:

  • Resolved: HA and DRS appear disabled when VM Storage profiles feature is enabled or disabled for a cluster.
    When VM storage profiles feature is enabled or disabled for a cluster, it causes a discrepancy in HA and DRS cluster configuration.
  • Resolved: File-based FDM logging can be enabled inadvertently for ESX 5.x hosts in a mixed cluster with ESX 5.x and ESX 4.x hosts.
    The default FDM logging behavior for ESX 5.x hosts is to use syslog, file-based logging is disabled. In a HA cluster with mixed of 5.x and pre-5.x hosts using DAS advanced option das.config.log.maxFileNum to increase number of log files on the pre-5.0 hosts will inadvertently enable file-based logging for ESX 5.x hosts. This can lead to ESX scratch partition to run out of space.
    This issue is resolved in this release by introducing HA cluster advanced parameter “das.config.log.outputToFiles”. To enable file-based logging for ESX 5.x hosts, both “das.config.log.maxFileNum” need to configure to a value greater than 2 and “das.config.log.outputToFiles” is equal to “true”.

ESXi 5.0 Update 1:

  • Resolved / New: No error message is logged when VMkernel stops a virtual machine on a datastore that is in PDL state
    When a SCSI device goes into permanent device loss (PDL) state, all the virtual machines that use datastores backed by that SCSI device are affected. Some third party HA solutions incorporate a VMX option where disk.terminateVMOnPDLDefault is set to True. With this option the VMkernel stops such affected virtual machines. Starting with this release, when VMkernel stops affected virtual machines, a warning message similar to the following is logged in vmkernel.log once for each virtual machine.
  • New: Enablement of session timeout to ESXi Tech Support Mode (TSM)
    After you log in to an ESXi host at the console and then log in to the Tech Support Mode (Mode) as root user and initiate a remote server console access session, a non-privileged user might obtain root access to the ESXi host, if the remote access session has not timed out or remains idle.Starting with this release, you can configure a session timeout to exit ESXi Tech Support Mode (TSM) as follows:

    1. Log in to Tech Support Mode (Mode) as root user.
    2. Edit /etc/profile file to add TMOUT=<timeout value in seconds>.
    3. Exit Tech Support Mode (Mode).

vShield 5.0.1:

  • New: vShield App High Availability enhancements automatically restarts vShield App or virtual machines if a heartbeat is not detected.
  • New: Enablement of Autodeploy (Stateless ESXi) by providing vShield VIBs (host modules) for download from vShield Manager.

 

Migrating VMs between clusters in vSphere 5.0 results in VMs being unprotected?

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Today on the community forums someone mentioned an issue where his VMs were not protected by vSphere HA after they had been migrated between clusters. After reading it I vague recalled this being a known issue. I dug up the KB and the work around is fairly simple:

  • Disable HA on the cluster where the unprotected VM resides
  • Enable HA on the cluster again

If you need to do a lot of migrations to a different cluster you can also temporarily disable HA, migrate all VMs and then enable it again. This leads to the same result as above, all VMs will be protected again. This is on the radar of our developers and they are working on fixing this in a future release.

HA Admission Control does not disallow HA initiated restarts

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I had a question about HA Admission Control today and as this is something that has come up multiple times I figured I would dedicate an article to it. This customer had enabled HA Admission Control and artificially wanted to control the amount of virtual machines a single host could run by manually specifying the slot size. (For more details on Admission Control slot sizes and how to configure these read the Deepdive page.) When they simulated a failure they were surprised that some host had more virtual machines running than should be allowed according to the configured slot size… This is however, contrary to their beliefs, by design. Let me copy/paste a paragraph from our book which talks about admission control.

What is HA Admission Control about? Why does HA contain this concept called Admission Control? The “Availability Guide” a.k.a HA bible states the following:

“vCenter Server uses admission control to ensure that sufficient resources are available in a cluster to provide failover protection and to ensure that virtual machine resource reservations are respected.”

Please read that quote again and especially the first two words. Indeed it is vCenter Server that is responsible for Admission Control. Although this might seem like a trivial fact it is important to understand that this means that Admission Control will not disallow HA initiated restarts. HA initiated restarts are done on a host level and not through vCenter. It is Admission Control’s task to ensure sufficient resources are available for HA to restart virtual machines, hence the reason HA does not take Admission Control in to account.

I hope this clears things up. I was pretty sure I have discussed this in multiple articles but as it comes up fairly often I figured dedicating and article to it would make it easier to find. I know it is not really clear in our documentation and I’ve requested this to be changed to reflect the actual behavior and avoid misunderstandings like these.

I selected “failover host” and my VMs still end up on a different host after an HA event

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I received a question today about HA admission control policies, and more specifically about the “failover host” admission control policy. The question was why VMs were restarted on a different host then selected with the “Failover Host” admission control policy. Shouldn’t this policy guarantee that a VM is restarted on the designated host?

The answer is fairly straight forward, and I thought I blogged about this already but I cannot find it so here it goes. Yes, in a normal condition HA will request the designated failover host to restart the failed VMs. However there are a couple of cases where HA will not restart a VM on the designated failover host(s):

  • When the failover host is not compatible with the virtual machine (portgroup or datastore missing)
  • When the failover host does not have sufficient resource available for the restart
  • When the virtual machine restart fails HA retries on a different host

Keep that in mind when using this admission control policy, it is no hard guarantee that the designated failover host will restart all failed VMs.