Manage ESXi Coredump Files

Okay, admit it, this is not a new topic, but it cost me some time in a client project. Since this blog also acts as a swap partition of my brain, I wrote it down for future reference. It is important to follow the steps correctly so that the changes are preserved after a reboot.

Why a Coredump-File?

Modern ESXi installations starting with version 7 use a new partition layout of the boot device. Coredumps are also located there. But only when the boot medium is not a USB flash medium and not an SD card. In such cases the coredump is relocated to a VMFS datastore with at least 32GB capacity.

This is exactly the case I found in a customer environment. The system was migrated from vSphere 6.7 and therefore still had the old boot layout on a ( at that time still fully supported) SD-Card RAID1. We found a vmkdump folder with files for each host on one of the shared VMFS datastores. This (VMFS5) datastore was supposed to be decommissioned and replaced with a VMFS6 datastore. (Side note from the VCI: there is no online migration path from VMFS5 to VMFS6) 😉 So the vmkdump files had to be removed from there.


First, we get an inventory of the coredump files.

esxcli system coredump file list

All coredump files of all ESXi hosts are listed here. Each line contains the path and the Active and Configured (true or false) states. Active means that this is the current coredump file of this host. It is important that the value for Configured also has the status ‘true’. Otherwise the setting will not survive a reboot. Only the coredump file of the current host has the status ‘active’. All other files belong to other hosts and are therefore active=false.

By default, the host chooses the first matching VMFS datastore. This is not necessarily the desired one.

Remove the current Coredump-File

First we delete the active coredump file of the host. We have to force the removal because it is set as active=true.

esxcli system coredump file remove --force

If we execute the list command from above again, there should be one line less.

Add a new Coredump File

The next command creates a new coredump file at the destination. If it does not already exist, a vmkdump folder is created and the dumpfile is created in it. We specify the desired file name without extension, because it will be created automatically (.dumpfile).

esxcli system coredump file add -d <Name | UUID> -f <filename>

Example: Name of the host is “ESX-01” and the VMFS datastore has the name “Service”. The datastore may be specified as either DisplayName or Datastore_UUID.

esxcli system coredump file add -d Service -f ESX-01

A folder vmkdump will be created on the designated datastore and a file named ESX-01.dumpfile will be created in it. We can check this using the list command.

esxcli system coredump file list

A new line will appear with the full path to the new dumpfile. However, the status is still active=false and configured=false. It might be useful to copy this full path to the clipboard, because it is required in the next step.

Activate Dumpfile

In the following step, we set the created dumpfile to active. This way, the setting is retained even after a host reboot. We specify the complete path to the dumpfile. The copy from the clipboard is helpful here and avoids typos.

esxcli system coredump file set -p <path_to_dumpfile>


esxcli system coredump file set -p /vmfs/volumes/<UUID>/vmkdump/ESX-01.dumpfile

A final List command validates the result.


vSAN Cluster Live-Migration to new vCenter instance

What can be done if the production vCenter Server appliance is damaged and you need to migrate a vSAN cluster to a new vCenter appliance?

In this post, I will show how to migrate a running vSAN cluster from one vCenter instance to a new vCenter under full load.

Anyone who works with vSAN will have a sinking feeling in their guts thinking about this. Why would one do such a thing? Wouldn’t it be better to put the cluster into maintenance mode? – In theory, yes. In practice, however, we repeatedly encounter constraints that do not allow a maintenance window in the near future.

Normally, vCenter Server appliances are solid and low-maintenance units. Either they work, or they are completely destroyed. In the latter case, a new appliance could be deployed and a configuration restore could be applied from the backup. None of this applied to a recent project. VCSA 6.7 was still working halfway, but key vSAN functionality was no longer operational in the UI. An initial idea to fix the problem with an upgrade to vCenter v7 and thus to a new appliance proved unsuccessful. Cross-vCenter migration of VMs (XVM) to a new vSAN cluster was also not possible, firstly because this feature was only available starting with version 7.0 update 1c, and secondly because only two new replacement hosts were available. Too few for a new vSAN cluster. To make things worse, the source cluster was also at its capacity limit.

There was only one possible way out: stabilize the cluster and transfer it to a new vCenter under full load.

There is an old, but still valuable post by William Lam on this topic. With this, and the VMware KB 2151610 article, I was able to work out a strategy that I would like to briefly outline here.

The process actually works because, once set up and configured, a vSAN cluster can operate autonomously from the vCenter. The vCenter is only needed for purposes of monitoring and configuration changes.

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Unclaim vSAN Disks in ESXi Host

While playing with the latest ESXi / vSAN beta, I ran into a problem. I was about to deploy a vCenter Server Appliance (VCSA) onto a single ESXi host, that was designated to become a vSAN Cluster. During initial configuration of vCenter something stalled. Needless to say that it’s been a DNS problem. 😉

That part of vCenter/vSAN deployment is delicate. If something goes wrong here, you have to start over again and deploy a new vCenter appliance. When you run the installer a second time (after you have fixed your DNS issues) you won’t see any disk devices to be claimed by vSAN. Where have they gone? Well, actually they are still there, but during the first deployment effort they were claimed by vSAN and now form a vSAN datastore. But a greenfield vSAN deployment on a first host needs disks that do not contain any vSAN or VMFS datastore.

How to release disks?

Usually you can remove Disk Groups in vCenter. But we don’t have a vCenter at this point. Looks like a chicken-and-egg problem. But we do have a host and a shell and esxcli. Start SSH service on the host and connect to the shell (e.g. Putty).

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