Snapshot Files and Limitations

When you take a snapshot, you capture the state of the virtual machine settings and the virtual disk. If you are taking a memory snapshot, you also capture the memory state of the virtual machine. These states are saved to files that reside with the base files of the virtual machines.

Snapshot Files

A snapshot consists of files that are stored on a supported storage device. A Take Snapshot operation creates .vmdk, -delta.vmdk or -sesparse.vmdk, .vmsd, and .vmsn files. By default, the first and all delta disks are stored with the base .vmdk file. The .vmsd and .vmsn files are stored in the virtual machine directory.

SEsparse is a default format for all delta disks on the VMFS6 datastores.

Delta disk files: A .vmdk file to which the guest operating system can write. The delta disk represents the difference between the current state of the virtual disk and the state that existed at the time that the previous snapshot was taken. When you take a snapshot, the state of the virtual disk is preserved, the guest operating system stops writing to it, and a delta or child disk is created.
A delta disk has two files. One is a small descriptor file that contains information about the virtual disk, such as geometry and child-parent relationship information. The other one is a corresponding file that contains the raw data.; The files that make up the delta disk are called child disks or redo logs.
Flat file: A -flat.vmdk file that is one of two files that comprises the base disk. The flat disk contains the raw data for the base disk. This file does not appear as a separate file in the Datastore Browser.
Database file: A .vmsd file that contains the virtual machine's snapshot information and is the primary source of information for the Snapshot Manager. This file contains line entries, which define the relationships between snapshots and between child disks for each snapshot.
Memory file: A .vmsn file that includes the active state of the virtual machine. Capturing the memory state of the virtual machine lets you revert to a powered on virtual machine state. With nonmemory snapshots, you can only revert to a powered off virtual machine state. Memory snapshots take longer to create than nonmemory snapshots. The time the ESXi host takes to write the memory onto the disk depends on the amount of memory the virtual machine is configured to use.

A Take Snapshot operation creates .vmdk, -delta.vmdk or -sesparse.vmdk, and .vmsn files and updates .vmsd file.

File Description

File	Description
The base disk names in a VM are: `vmname`_`number`.vmdk and `vmname`_`number`-flat.vmdk The newly created disk names in a VM are: `vmname`_`number`-`number`.vmdk , `vmname`_`number`-`number`-sesparse.vmdk or `vmname`_`number`-`number`-delta.vmdk	Snapshot file that represents the difference between the current state of the virtual disk and the state that existed at the time the previous snapshot was taken. The base disk filename uses the following syntax, `vm_000001.vmdk` where vm is the name of the virtual machine and the six-digit number, 000001, is based on the files that already exist in the directory. The number does not consider the number of disks that are attached to the virtual machine. The newly created disk filenames are, for example: `test-512-000001-sesparse.vmdk` `test-512-000001.vmdk` `test-512_1-000001-sesparse.vmdk` `test-512_1-000001.vmdk`
`vmname`.vmsd	Database of the virtual machine's snapshot information and the primary source of information for the Snapshot Manager.
`vmname`-Snapshot`number`.vmsn	Memory state of the virtual machine at the time you take the snapshot. The filename uses the following syntax, `vm1-Snapshot1.vmsn`, where vm1 is the virtual machine name, and Snapshot1 is the first snapshot. Note: A .vmsn file is created each time you take a snapshot, regardless of the memory selection. A .vmsn file without memory is much smaller than one with memory.

The base disk names in a VM are:

vmname_number.vmdk and vmname_number-flat.vmdk

The newly created disk names in a VM are:

vmname_number-number.vmdk , vmname_number-number-sesparse.vmdk or vmname_number-number-delta.vmdk

Snapshot file that represents the difference between the current state of the virtual disk and the state that existed at the time the previous snapshot was taken.

The base disk filename uses the following syntax, vm_000001.vmdk where vm is the name of the virtual machine and the six-digit number, 000001, is based on the files that already exist in the directory. The number does not consider the number of disks that are attached to the virtual machine.

The newly created disk filenames are, for example:

test-512-000001-sesparse.vmdk
test-512-000001.vmdk
test-512_1-000001-sesparse.vmdk
test-512_1-000001.vmdk

vmname.vmsd Database of the virtual machine's snapshot information and the primary source of information for the Snapshot Manager.

vmname-Snapshotnumber.vmsn

Memory state of the virtual machine at the time you take the snapshot. The filename uses the following syntax, vm1-Snapshot1.vmsn, where vm1 is the virtual machine name, and Snapshot1 is the first snapshot.

Note: A .vmsn file is created each time you take a snapshot, regardless of the memory selection. A .vmsn file without memory is much smaller than one with memory.

Snapshot Limitations

Snapshots can affect the virtual machine performance and do not support some disk types or virtual machines configured with bus sharing. Snapshots are useful as short-term solutions for capturing point-in-time virtual machine states and are not appropriate for long-term virtual machine backups.

VMware does not support snapshots of raw disks, RDM physical mode disks, or guest operating systems that use an iSCSI initiator in the guest.
Virtual machines with independent disks must be powered off before you take a snapshot.
You can take a memory snapshot of a virtual machine with an independent disk only to analyze the guest operating system behavior of a virtual machine. You cannot use such snapshots for virtual machine backups because restoring this type of snapshots is unsupported.
Quiesced snapshots require VMware Tools installation and guest operating system support.
Snapshots are not supported with PCI vSphere DirectPath I/O devices.
VMware does not support snapshots of virtual machines configured for bus sharing. If you require bus sharing, consider running backup software in your guest operating system as an alternative solution. If your virtual machine currently has snapshots that prevent you from configuring bus sharing, delete (consolidate) the snapshots.
Snapshots provide a point-in-time image of the disk that backup solutions can use, but Snapshots are not intended for a robust method of backup and recovery. If the files containing a virtual machine are lost, its snapshot files are also lost. Also, large numbers of snapshots are difficult to manage, consume large amounts of disk space, and are not protected if there is hardware failure.
Snapshots can negatively affect the performance of a virtual machine. Performance degradation is based on how long the snapshot or snapshot tree is in place, the depth of the tree, and how much the virtual machine and its guest operating system have changed from the time you took the snapshot. Also, you might see a delay in the amount of time it takes the virtual machine to power on. Do not run production virtual machines from snapshots on a permanent basis.