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VMware Virtual Machine File types and extensions

January 24, 2017

VMware Virtual Machine File types and extensions

Today we are going to talk about the VMware Virtual Machine File types and extensions used. 

Fig 1.1- VMware VM File Types & Extensions

*.vmx file: 
A virtual machine's primary configuration file resides in this file. The virtual machine's hardware settings are stored in this file when you create it and configure its settings. 

A text file containing information about hard disks, network adapters, memory, CPU, ports, power options, etc. Depending on what you wish to add, you can edit these files directly or use the VMware GUI (Edit Settings on the VM) which will automatically update them.

*.nvram file: 
The CMOS/BIOS file contains information about the virtual machine. PhoenixBIOS 4.0 Release 6 is a well-known and highly successful BIOS that is fully compatible with all the major standards, including USB, PCI, ACPI, 1394, WfM and PC2001.

When the VM is powered on, the NVRAM file will be recreated if it is deleted or missing. In this file are saved all BIOS changes made through the Setup program (F2 during boot).They usually have a size of less than 10K and are in a binary format (not text).

vmdk files:
These are the disk files that are created for each virtual hard drive in your VM. There are 3 different types of files that use the vmdk extension, they are:

  • *flat.vmdk file: Every virtual hard drive has a raw disk file. Virtual machine data comprises most of a .vmdk file, with a small portion devoted to virtual machine overhead. In terms of size, it'll be similar to your virtual hard drive.
  • *.vmdk file: The raw data is no longer contained in this file. This is actually the disk descriptor file that describes the size and geometry of the virtual disk file. The file contains information about –flat.vmdk and hard drive adapter types, drive sectors, heads and cylinders, etc. It is in text format and carries the name of the file it is associated with. Virtual hard drives assigned to your virtual machines will each contain one of these files. Open the file and look at the Extent Description field to determine which –flat.vmdk file it corresponds to.
  • *delta.vmdk file: This is the differential file created when you take a snapshot of a VM (also known as REDO log). When you snapshot a VM it stops writing to the base vmdk and starts writing changes to the snapshot delta file. The snapshot delta will initially be small and then start growing as changes are made to the base vmdk file, The delta file is a bitmap of the changes to the base vmdk thus is can never grow larger than the base vmdk. A delta file will be created for each snapshot that you create for a VM. These files are automatically deleted when the snapshot is deleted or reverted in snapshot manager.

*.vswp file:
This is the VM swap file (earlier ESX versions had a per host swap file) and is created to allow for memory over commitment on a ESX server. The file is created when a VM is powered on and deleted when it is powered off. 

By default when you create a VM the memory reservation is set to zero, meaning no memory is reserved for the VM and it can potentially be 100% overcommitted. As a result of this a vswp file is created equal to the amount of memory that the VM is assigned minus the memory reservation that is configured for the VM. 

So a VM that is configured with 2GB of memory will create a 2GB vswp file when it is powered on, if you set a memory reservation for 1GB, then it will only create a 1GB vswp file. If you specify a 2GB reservation then it creates a 0 byte file that it does not use. 

When you do specify a memory reservation then physical RAM from the host will be reserved for the VM and not usable by any other VM’s on that host. A VM will not use it vswp file as long as physical RAM is available on the host. 

Once all physical RAM is used on the host by all its VM’s and it becomes overcommitted then VM’s start to use their vswp files instead of physical memory. Since the vswp file is a disk file it will effect the performance of the VM when this happens. 

If you specify a reservation and the host does not have enough physical RAM when the VM is powered on then the VM will not start.

*.vmss file:
This file is created when a VM is put into Suspend (pause) mode and is used to save the suspend state. It is basically a copy of the VM’s RAM and will be a few megabytes larger than the maximum RAM memory allocated to the VM. 

If you delete this file while the VM is in a suspend state It will start the VM from a normal boot up instead of starting the VM from the state it was when it was suspended. This file is not automatically deleted when the VM is brought out of Suspend mode. 

Like the vswp file this file will only be deleted when the VM is powered off (not rebooted). If a Vmss file exists from a previous suspend and the VM is suspended again then the previous file is re-used for the subsequent suspensions. 

Also note that if a vswp file is present it is deleted when a VM is suspended and then re-created when the VM is powered on again. The reason for this is that the VM is essentially powered off in the suspend state, it’s RAM contents are just preserved in the vmss file so it can be quickly powered back on.

*.vmxf file:
This is a supplemental configuration file in text format for virtual machines that are in a team. Note that the .vmxf file remains if a virtual machine is removed from the team. 

Teaming virtual machines is a Vmware Workstation feature and includes the ability to designate multiple virtual machines as a team, which administrators can then power on and off, suspend and resume as a single object making it particularly useful for testing client-server environments. 

This file still exists with ESX server virtual machines but only for compatibility purposes with Workstation.

*.vmsd file:
This file is used to store metadata and information about snapshots. This file is in text format and will contain information such as the snapshot display name, uid, disk file name, etc. 

It is initially a 0 byte file until you create your first snapshot of a VM and from that point it will populate the file and continue to update it whenever new snapshots are taken. This file does not cleanup completely after snapshots are taken. 

Once you delete a snapshot it will still leave the fields in the file for each snapshot and just increment the uid and set the name to “Consolidate Helper” presumably to be used with Consolidated Backups

*.vmsn file:
This is the snapshot state file, which stores the exact running state of a virtual machine at the time you take that snapshot. This file will either be small or large depending on if you select to preserve the VM’s memory as part of the snapshot. 

If you do choose to preserve the VM’s memory then this file will be a view megabytes larger then the maximum RAM memory allocated to the VM. This file is similar to the vmss (Suspend) file. A vmsn file will be created for each snapshot taken on the VM, these files are automatically deleted when the snapshot is removed.

*.log file:
This is the file that keeps a log of the virtual machine activity and is useful in troubleshooting virtual machine problems. Every time a VM is powered off and then back on a new log file is created. The current log file for the VM is always vmware.log. 

The older log files are incremented with a -# in the filename and up to 6 of them will be retained. (i.e.. vmware-4.log) The older .log files are always removal at will, the latest .log file can be deleted when the VM is powered off.