Yellow Bricks

by Duncan Epping

vcenter

RE: VMware vCenter 4 minimum RAM requirement, is it 2GB or 3GB?

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Simon Long just posted a new article which is titled; VMware vCenter 4 minimum RAM requirement, is it 2GB or 3GB? Simon found two answers(2GB vs 3GB) in three documents which are listed below:

  1. VMware’s ESX and vCenter Server Installation Guide
  2. Scott Lowe’s – Mastering VMware vSphere 4.0
  3. VMware Education – VMware vSphere 4 Install, Configure, and Manage course books

Now for me the answer is simple. Anything that’s stated in an official VMware document is leading so that would rule out Scott. So what about the course material vs the install guide cause both are VMware documents. Course material usually doesn’t get updated as often as it should. Don’t use course material for specific limits or requirements. Use the official product documentation. Course material should be used to understand the concepts, for specifics use the documentation!

All vSphere documentation can be found here: http://www.vmware.com/support/pubs/vs_pages/vsp_pubs_esx40_vc40.html

These docs should be leading at all times!

HA Admission Control and DPM

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A couple of days ago we had a discussion on Admission Control and DPM internally at VMware. One of our customers had enabled DPM on a HA cluster. During the evening 4 out of 5 hosts where placed into standby mode because of this.

This customer, as many of our customers have these days, had vCenter running virtual. This of course led to the question; what happens if this one host fails and our virtual vCenter server is running on it?
That’s an easy one; nothing. It might not be the answer you are looking for but when the host fails that runs vCenter there’s no host or service left to get these hosts out of maintenance mode or restart your VMs.

Now maybe even more important; what causes this behavior?
This behavior is caused by the fact that admission control is disabled. If you disable admission control DPM will put hosts into standby mode even if it violates failover requirements. This means that if you have virtualized your vCenter server this is definitely something to be aware of.

For more info/background: http://kb.vmware.com/kb/1007006

Automatic rescan of your HBAs….

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As some of you, and I hope all of you, have noticed by now when you create / expand / extent / delete a datastore a rescan of your HBAs will automatically be initiated. This however can lead to a “rescan storm” when you are building a new environment.

You can imagine that it’s pointless to rescan your HBA 25 times in a row when you are adding more than 1 new datastore. I can even imagine you would like to be in control; when, which server and at what time. This behavior was introduced with vCenter 2.5 U2 I believe but as I just found out can be disabled. Now keep in mind that disabling is not a best practice. This should be avoided as a default setting but will come in handy when you are building a new site.

  1. Open up the vSphere Client
  2. Go to Administration -> vCenter Server
  3. Go to Settings -> Advanced Settings
  4. If the key “config.vpxd.filter.hostRescanFilter” is not available add it and set it to false

Make sure to set it to “true” as soon as you are done because you would like to make sure the environment is consistent in the future when you or your customer is adding/removing/expanding a datastore.

Second vswp file when doing a VMotion with vSphere?

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I was just reading this topic on the VMTN community. In short, a second vswp file gets created during a VMotion. As the starter of this topic noticed it could lead to not being able to VMotion VMs if you don’t have enough free disk space on your VMFS volume.

One of my UK colleagues, David Burgess, jumped in and explained what is happening during the VMotion and why this temporary vswp file is being created. Read it, it’s useful info:

  1. It is only used if the target is under memory pressure. It is thin provisioned so even though it looks the size of the memory it should have very little impact on the free space of the VMFS.
  2. The other thing is that the temp swap will only be used for activity as the machine transitions so should not grow to the size of the memory. If you “du” the file systems you should see the the blocks being consumed. Engineers think this should be tops 400M, if it is used at all. By pressured we mean the amount of memory free is low. That will not deny the VM to VMotion unless we can’t allocate enough reserved memory (this is zero by default). Once the transition is complete the VM reverts to the original swap file and the temp is deleted.

Take a look at the screenshot David uploaded, the bottom two vswp files are the ones created during the VMotion and as you can see are consuming 0 blocks.