vBlock Tip: Set VMFS3.MaxHeapSizeMB to 256MB on all ESXi hosts


This is the sort of issue I came across this week that you would expect VCE to make a de facto standard in the vBlock.

Why? Because the performance hit is negligible (a slight increase in additional kernel memory of 64mb), vBlock customers are likely to hit this ceiling and its another setting that we then don’t have to worry about.

I started running into issues vMotioning two VMs. It turns out this is a known issue as per KB1004424.

I was told by VMware: ‘ESXi 5.0 Host(Source on which you are trying to power on the VM)  already has 18 virtual disks(.vmdk) greater than 256GB in size open and you are trying to power on a virtual machine with another virtual disk of greater than 256GB in size.’

The heap size is effectively the amount of VMDK storage that can be hosted, across all virtual machines, for a given host.

The default heap size is a value of 80. To calculate the amount of open VMDK storage on the host that is available,  multiply 80 x 256 * 1024 — A 80MB heap value extends to 20 TB of open VMDK storage on a host.

Increasing the size to 256 results in 256 x 256 * 1024 — A 256MB heap value extends to 64 TB of open VMDK storage on a host, which should be plenty.

I was provided the following instructions which I repeated on each host to fix:

  1. Login to the vCenter Server or the ESXi host using the vSphere Client.
  2. Click on the configuration tab of the ESXi Host
  3. Click on Advanced Settings under Software
  4. Select VMFS3
  5. Change the value of VMFS3.MaxHeapSizeMB to 256
  6. Click on OK
  7. Reboot host

After rebooting each host the problem was solved. That was an easy one, for once!

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3 responses to “vBlock Tip: Set VMFS3.MaxHeapSizeMB to 256MB on all ESXi hosts

  1. Hi, Great information, thanks! Do you happen to know whether this 64TB limitation applies to NFS storage as well? Or to RDMs?

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