Yellow Bricks

by Duncan Epping

5.0

Using a CNAME (DNS alias) to mount an NFS datastore

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I was playing around in my lab with NFS datastores today. I wanted to fail-over a replicated NFS datastore without the need to re-register the virtual machines running on them. I had mounted the NFS datastore using the IP address and as that is used to create the UUID it was obvious that it wouldn’t work. I figured there should be a way around it but after a quick search on the internet I still hadn’t found anything yet.

I figured it should be possible to achieve this using a CNAME but also recalled something around vCenter screwing this up again. I tested it anyway and with success. This is what I did:

  • Added both NFS servers to DNS
  • Create a CNAME (DNS Alias) and pointed to the “active” NFS server
    • I used the name “nasdr” to make it obvious what it is used for
  • Created an NFS share (drtest) on the NFS server
  • Mount the NFS export using vCenter or though the CLI
    • esxcfg-nas -a -o nasdr -s /drtest drtest
  • Check the UUID using vCenter or through the CLI
    • ls -lah /vmfs/volumes
    • example output:
      lrwxr-xr-x    1 root     root           17 Feb  6 10:56 drtest -> e9f77a89-7b01e9fd
  • Created a virtual machine on the nfsdatastore
  • Enabled replication to my “standby” NFS server
  • I killed my “active” NFS server environment (after validating it had completed replication)
  • Changed the CNAME to point to the secondary NFS server
  • Unmounted the volume old volume
    • esxcfg-nas -d drtest
  • I did a vmkping to “nasdr” just to validate the destination IP had changed
  • Rescanned my storage using “esxcfg-rescan -A”
  • Mounted the new volume
    • esxcfg-nas -a -o nasdr -s /drtest drtest
  • Checked the UUID using the CLI
    • ls -lah /vmfs/volumes
    • example output:
      lrwxr-xr-x    1 root     root           17 Feb  6 13:09 drtest -> e9f77a89-7b01e9fd
  • Powered on the virtual machine now running on the secondary NFS server

As you can see, both volumes had the exact same UUID. After the fail-over I could power-on the virtual machine. No need to re-register the virtual machines within vCenter first. Before I wanted to share it with the world I reached out to my friends at NetApp. Vaughn Stewart connected me with Peter Learmonth who validated my findings and actually pointed me to a blog article he wrote about this topic. I suggest to head-over to Peter’s article for more details on this.

Setting the default affinity rule for Storage DRS

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On my blog article for yesterday “Rob M” commented that the default affinity rule for Storage DRS (SDRS), keep VM files together, did not make sense to him. One of the reasons this affinity rule is set is because customers indicated that from an operational perspective it would be easier if all files of a given VM (vmx / vmdk’s) would reside in the same folder. Especially troubleshooting was one of the main reasons, as this lowers complexity. I have to say that I fully agree with this, I’ve been in the situation where I needed to recover virtual machines and having them spread across multiple datastore really complicates things.

But, just like Rob, you might not agree with this and rather have SDRS handling balancing on a file per file basis. That is possible and we documented this procedure in our book. I was under the impression that I blogged this, but just noticed that somehow I never did. Here is how you change the affinity rule for the current provisioned VMs in a datastore cluster:

  1. Go to Datastores and Datastore Clusters
  2. Right click a datastore cluster and select “edit settings”
  3. Click “Virtual machine settings”
  4. Deselect “Keep VMDKs together”
    1. For virtual machines that need to stick together you can override the default by ticking the tick box next to the VM


Also check out this article by Frank about DRS/SDRS affinity rules, useful to know!

How cool and useful is Storage DRS?!

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I was just playing around in my lab and created a whole bunch of VMs when I needed to deploy to large virtual machines. Both of them had 500GB disks. The first one deployed without a hassle, but the second one was impossible to deploy, well not impossible for Storage DRS. Just imagine you had to figure this out yourself! Frank wrote a great article about the logic behind this and there is no reason for me to repeat this, just head over to Frank’s blog if you want to know more..

And the actually migrations being spawned:

Yes, this is the true value of Storage DRS… initial placement recommendations!

Creating an IP-Pool for VC Ops

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I was importing the VC Ops virtual appliance and during the import I got a question around IP addresses. So I figured I would enter two IP addresses and that would be it. As soon as I powered on the VM I received the following error:

Cannot initialize property ‘vami.netmask0.VM_1’ since network ‘VM Network’ has no associated IP Pools configuration.

I figured this would be simple so I jumped back to “home” and went to the network section… Nothing around IP Pools. Even on a host or cluster layer there was nothing. Luckily my colleague Cormac jumped in and said check the “Datacenter” object, there should be an IP Pool tab there. He was right. Weird place and definitely something that needs to be improved. Anyway, configuring an IP Pool itself, now that I found it, was easy:

  1. Click your Datacenter object
  2. Go to the “IP Pools” tab
  3. Click “Add”
  4. Fill out the details:
    1. Subnet: which network will be used and what is the mask? (You can use a subnet calculator if you don’t know…)
    2. Enter the details of the gateway
    3. Specify a range, the format is “10.1.1.10#10”, this would result in a range from 10.1.1.10 until 10.1.1.19 (10 addresses counting from .10)
    4. Don’t forget to tick the “Enable IP Pool” check box
    5. Click on the “Associations” Tab and associate it to a network!
    6. Also, fill out the DNS and proxy details if and when required.
  5. This is what it should look like:

It is as simple as that, but indeed not easy to find hence the reason I figured a short article was in place.

PS: Creating a range and enabling the “IP Pool” is not required. “Enable IP Pool” enables the use of the Range. In this example I had to use a range as I could only use a specific range of this subnet.

 

Re: when to disable HA? /cc @hashmibilal

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Bilal Hashmi wrote a nice article about HA today and in this article he asked a couple of questions. As I think the info is useful for everyone I decided to respond through a blog article instead of by commenting.

Let me start by saying that in general HA should never be disabled. The later versions of vSphere have a neat option called “Enable Host Monitoring”. This option should be used for scheduled network maintenance. The difference between disabling host monitoring and disabling HA is that disabling host monitoring does not cause a full reconfiguration (see screenshot below) of HA and a new election process. Just the “host monitoring” functionality is disabled, which is what you want in this scenario.

Bilal asked multiple questions / made multiple statements in his article, I will respond to two of these specifically to explain the way HA handles failures/isolation:

In this case within 30 sec of the management network outage, each host would have declared itself isolated and wont attempt to restart any VMs like the primaries would in vSphere 5.

So why is this? As soon as a Master is isolated it will drop “ownership” of datastores on which VMs are running that are part of its cluster. Before the other hosts trigger the isolation response for a given VM they will validate if the datastore on which this VM is stored is “owned” by a master. In the case of a cluster wide isolation due to a network outage / maintenance the ownership would be dropped and this would result in HA not triggering the isolation response. This is a major change compared to vSphere 4.x and prior!

Now what happens when the network outage is over and the hosts are in a position to talk to each other? I have not been able to find documentation on whether an isolated host will enter an election (vSphere 4 or 5) ones the communication channel is open and bring the cluster back to life.

Lets focus on vSphere 5.0 as that seems most relevant. A host remains isolated until it observes HA network traffic, like for instance election messages OR it starts getting a response from an isolation address. Meaning that as long as the host is in “isolated state” it will continue to validate its isolation by pinging the isolation address. As soon as the isolation address responds it will initiate an election process or join an existing election process and the cluster will return to a normal state.

There’s absolutely no need to manually intervene. HA takes care of all of this for you.