Virtual disk files, commonly known as VMDK (Virtual Machine Disk) files, are a fundamental component of virtualized environments. These files, with the .vmdk extension, serve as storage volumes for virtual machines, housing the guest operating system’s file system. This article will provide an overview of VMDK files, their types, and the different physical storage options available, along with the various additional files that make up a complete virtual machine configuration.
What is a VMDK File?
A VMDK file is a container format used by VMware to encapsulate the contents of a virtual hard drive. It simulates the functionality of a physical hard disk, allowing the guest OS to read from and write to it as if it were a real disk. VMDK files can be stored on a variety of physical storage devices, depending on the underlying infrastructure and the type of virtualization platform being used.
Physical Disk Types
1. Managed Disks (Managed Disk)
- Managed disks typically reside on VMFS (Virtual Machine File System) formatted storage.
- They support connection to ESX/ESXi hosts via SAN (Storage Area Network), iSCSI, or SAS (Serial Attached SCSI).
- In a vCenter environment, VMDK files are stored on shared datastores, enabling easy migration between hosts without moving the VMDK files.
- For ESX/ESXi hosts, these files are usually found within the `/vmfs/volumes` directory.
2. Hosted Disks (Hosted Disk)
- Hosted disks do not require a specific file system format and adapt to the native disk type of the host.
- In Workstation environments, VMDK files are stored directly on the local disk, using the host’s file system.
Creating Virtual Disk Types
When creating a virtual disk, you have several options regarding how space is allocated and initialized:
Lazy Zeroed Thick (Thick Provision Lazy Zeroed)
- Space is immediately allocated, but the actual zeroing out of the data is delayed until the first write operation.
- Provides good performance and is suitable for pool-based virtual desktops.
Eager Zeroed Thick (Thick Provision Eager Zeroed)
- Space is allocated and all data is zeroed out at creation time, which takes longer but offers the best performance.
- Ideal for applications requiring high I/O performance and for features like Fault Tolerance.
Thin Provision (Thin)
- Initially consumes only the space required, growing as more data is written, up to a maximum defined size.
- Efficient in terms of initial space usage, but may experience performance degradation under heavy I/O workloads.
VixDiskLib Supported Disk Types
VixDiskLib, a library for managing virtual disks, supports various disk types that cater to different use cases, such as those for vCenter and ESX/ESXi. The supported types include monolithic sparse, monolithic flat, split sparse, and split flat for hosted disks, and VMFS flat, VMFS sparse, and VMFS thin for managed disks, among others.
Additional Virtual Machine Files
In addition to the VMDK file, a VMware virtual machine includes a variety of other files, each serving a distinct purpose:
.vmx Configuration file for the virtual machine.
- -flat.vmdk Contains the actual data of the virtual disk.
- -ctk.vmdk Change tracking file, used for identifying changes since the last snapshot.
- .vmem Memory dump file, containing the state of the virtual machine’s memory.
- .vmss Suspended state information, used when the virtual machine is paused.
- .vmsd Metadata for snapshots, including snapshot names, identifiers, and associated disk files.
- .vmtx Template file, used for cloning new virtual machines.
- .nvram BIOS settings for the virtual machine.
- .vswp Swap file, used for virtual memory management.
- .log Log files, capturing operational information about the virtual machine.
- -s<>.vmdk / -f<>.vmdk Split VMDK files, either sparse or flat, divided into 2GB chunks.
- Snapshot.vmsn Snapshot state information, holding the state of the virtual machine at the time of the snapshot.
Conclusion
Understanding the nuances of VMDK files and the various components that make up a virtual machine is essential for effective virtualization management. By choosing the appropriate disk type and properly configuring the associated files, administrators can optimize performance, manage storage efficiently, and ensure the reliability of their virtualized infrastructure.