What is a snapshot?
A snapshot preserves the state and data of a virtual machine at a specific point in time.
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State includes the virtual machine’s power state (powered‐on, powered‐off, suspended, etc).
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Data
includes all the files that make‐up the virtual machine, including
disks, memory, and other devices, such as virtual network interface
cards.
A
virtual machine provides several operations for creating and managing
snapshots and snapshot chains. These operations let you create
snapshots, revert to any snapshot in the chain, and remove
snapshots. You can create extensive snapshot trees that you can use to save virtual machine state at any point in time, and restore it later, if necessary.
When creating a snapshot, there are several options you can specify:
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Name – used to identify the snapshot.
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Description – used to describe the snapshot.
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Memory – If the
flag is 1 or true, a dump of the internal state of the virtual machine
is included in the snapshot. Memory snapshots take longer to create.
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Quiesce – If the
flag is 1 or true, and the virtual machine is powered on when the
snapshot is taken, VMware Tools is used to quiesce the file system in
the virtual machine. Quiescing a file system is a process of bringing
the on‐disk data of a physical or virtual computer into a state
suitable for backups. This process might include such operations as
flushing dirty buffers from the operating systems in‐memory cache to
disk, or other higher‐level application‐specific tasks.
Note: Quiescing
indicates pausing or altering the state of running processes on a
computer, particularly those that might modify information stored on
disk during a backup, to guarantee a consistent and usable backup.
When a snapshot is created, it is comprised of the following files:
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-.vmdk and --delta.vmdk
A collection of .vmdk and -delta.vmdk
files for each virtual disk is connected to the virtual machine at the
time of the snapshot. These files can be referred to as child disks,
redo logs, and delta links. These child disks can later be considered
parent disks for future child disks. From the original parent disk,
each child constitutes a redo log pointing back from the present state
of the virtual disk, one step at a time, to the original.
Note: The
value may not be consistent across all child disks from the same
snapshot. The file names are chosen based on filename availability.
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.vmsd
The .vmsd
file is a database of the virtual machine's snapshot information and
the primary source of information for the snapshot manager. The file
contains line entries which define the relationships between snapshots
as well as the child disks for each snapshot.
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Snapshot.vmsn
These files are the memory state at the time of the snapshot.
What products use the snapshot feature?
In
addition to being able to use snapshot manager to create snapshots,
snapshots are used by many VMware and third party products and
features. Some VMware products that use snapshots extensively are:
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VMware Consolidated Backup
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VMware Data Recovery
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VMware Lab Manager
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VMware vCenter and the VMware Infrastructure Client (Snapshot Manager, Storage vMotion)
Note: This is not an exhaustive list and does not include third party programs.
How do snapshots work?
Our VMware API
allows VMware and third party products to perform operations with
virtual machines and their snapshots. Below is a list of common
operations that can be performed on virtual machines, snapshots or
using our API:
- CreateSnapshot – Creates a new snapshot of a virtual machine. As a side effect, this updates the current snapshot.
- RemoveSnapshot – Removes a snapshot and deletes any associated storage.
- RemoveAllSnapshots
– Remove all snapshots associated with a virtual machine. If a virtual
machine does not have any snapshots, then this operation simply returns
successfully.
- RevertToSnapshot – Changes the execution state of a virtual machine to the state of this snapshot.
The following
is a high level overview of how to create, remove, or revert snapshot
requests are processed within the VMware environment:
- A request to
create, remove, or revert a snapshot for a virtual machine is sent from
the client to the server using the VMware API.
- The request is forwarded to the VMware ESX host which is currently hosting the virtual machine in question.
Note: This only occurs if the original request was sent to a different server such as vCenter which is managing the ESX host.
- If the snapshot includes the memory option, the ESX host writes the memory of the virtual machine to disk.
Note:
The length of 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.
- If the
snapshot includes the quiesce option, the ESX host requests the guest
operating system to quiesce the disks via the VMware tools.
Note:
Depending on the guest operating system, the quiescing operation can be
done by the sync driver, the vmsync module, or Microsoft's Volume
Shadow Copy (VSS) service. For more information on quiescing, see for VSS or A virtual machine can freeze under load when you take quiesced snapshots or use custom quiescing scripts (5962168) for the SYNC driver.
- The ESX host makes the appropriate changes to the virtual machine's snapshot database (.vmsd file) and the changes are reflected in the snapshot manager of the virtual machine.
Note:
The operation to remove the snapshot entity in the snapshot manager
completes before the changes are made to the child disks. The snapshot
manager does not contain any snapshot entries while the virtual machine
continues to run from the child disk. For more information, see .
- The ESX host calls a function similar to the Virtual Disk API functions to make changes to the child disks (-delta.vmdk and .vmdk files) and the disk chain.
Note:
During a snapshot removal, if the child disks are large in size, the
operation may take a long time. This could result in a timeout error
message from either VirtualCenter or the VMware Infrastructure Client.
For more information about timeout error messages, see .
The child disk
The child disk
which is created with a snapshot, is a sparse disk. Sparse disks employ
the copy‐on‐write (COW) mechanism, in which the virtual disk contains
no data in places, until copied there by a write. This optimization
saves storage space. The grain is the unit of measure in which the
sparse disk uses the copy-on-write mechanism. Each grain is a block of
sectors containing virtual disk data. Default size is 128 sectors or
64KB.
Child disks and disk usage
The following are points which should be noted regarding the space utilization of Child disks:
- If a virtual machine is running off of a snapshot, it is making changes to a child or sparse disk. The more write operations made to this disk, the larger it grows.
- The space
requirements of the child disk are in addition to the parent disk on
which it depends. If a virtual machine has a 10 GB disk with a child
disk, the space being used will be 10 GB + the child disk size.
- Child disks have been known to grow large enough to fill an entire datastore.
- The speed at which child disks grow are directly dependent on the amount of I/O being done to the disk.
- The size of
the child disk has a direct impact on the length of time it takes to
delete the snapshot associated to the child disk.
The following Knowledge Base articles touch on the topic of child disks and disk usage:
The disk chain
Generally, when
you create a snapshot for the first time, the first child disk is
created from the parent disk. Successive snapshots generate new child
disks from the last child disk on the chain. The relationship can
change if you have multiple branches in the snapshot chain.
The following diagram is an example of a snapshot chain, each square represents a block of data or a grain as described above:
Caution: Manually
manipulating the individual child disks or any of the snapshot
configuration files may compromise the disk chain. VMware does not
recommend manually modifying the disk chain as it may result in data
loss. For more information, see .
