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 virtual machine's base files.
A snapshot consists of files that are stored on a supported storage device. A Take Snapshot operation creates .vmdk, -delta.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.
- Delta disk files
.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, which prevents the guest operating system from writing to it, and a delta or child disk is created.
A delta disk has two files, including a descriptor file that is small and contains information about the virtual disk, such as geometry and child-parent relationship information, and a corresponding file that contains the raw data.
- The files that make up the delta disk are referred to as child disks or redo logs. A child disk is a sparse disk. Sparse disks use the copy-on-write mechanism, in which the virtual disk contains no data in places, until copied there by a write operation. This optimization saves storage space. A grain is the unit of measure in which the sparse disk uses the copy-on-write mechanism. Each grain is a block of sectors that contain virtual disk data. The default size is 128 sectors or 64KB.
- 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 turned on virtual machine state. With nonmemory snapshots, you can only revert to a turned off virtual machine state. Memory snapshots take longer to create than nonmemory snapshots. The time the ESX host takes to write the memory onto the disk is relative to the amount of memory the virtual machine is configured to use.
A Take Snapshot operation creates .vmdk, -delta.vmdk, vmsd, and vmsn files.
|vmname-number.vmdk and vmname-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 filename uses the following syntax,
|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 file name uses the following syntax,
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.