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Each snapshot creates an additional delta .vmdk disk file. When you take a snapshot, the snapshot
mechanism prevents the guest operating system from writing to the base .vmdk file and instead directs all
writes to the delta disk file. The delta disk represents the difference between the current state of the virtual
disk and the state that existed at the time that you took the previous snapshot. If more than one snapshot
exists, delta disks can represent the difference between each snapshot. Delta disk files can expand quickly
and become as large as the entire virtual disk if the guest operating system writes to every block of the
virtual disk.
Taking Snapshots of a Virtual Machine
You can take one or more snapshots of a virtual machine to capture the settings state, disk state, and
memory state at different specific times. When you take a snapshot, you can also quiesce the virtual machine
files and exclude the virtual machine disks from snapshots.
When you take a snapshot, other activity that is occurring in the virtual machine might affect the snapshot
process when you revert to that snapshot. The best time to take a snapshot from a storage perspective, is
when you are not incurring a large I/O load. The best time to take a snapshot from a service perspective is
when no applications in the virtual machine are communicating with other computers. The potential for
problems is greatest if the virtual machine is communicating with another computer, especially in a
production environment. For example, if you take a snapshot while the virtual machine is downloading a
file from a server on the network, the virtual machine continues downloading the file and communicating
its progress to the server. If you revert to the snapshot, communications between the virtual machine and
the server are confused and the file transfer fails. Depending on the task that you are performing, you can
create a memory snapshot or you can quiesce the file system in the virtual machine.
Memory Snapshots
The default selection for taking snapshots. When you capture the virtual
machine's memory state, the snapshot retains the live state of the virtual
machine. Memory snapshots create a snapshot at a precise time, for example,
to upgrade software that is still working. If you take a memory snapshot and
the upgrade does not complete as expected, or the software does not meet
your expectations, you can revert the virtual machine to its previous state.
When you capture the memory state, the virtual machine's files do not
require quiescing. If you do not capture the memory state, the snapshot does
not save the live state of the virtual machine and the disks are crash
consistent unless you quiesce them.
Quiesced Snapshots
When you quiesce a virtual machine, VMware Tools quiesces the file system
of the virtual machine. A quiesce operation ensures that a snapshot disk
represents a consistent state of the guest file systems. Quiesced snapshots are
appropriate for automated or periodic backups. For example, if you are
unaware of the virtual machine's activity, but want several recent backups to
revert to, you can quiesce the files.
If the virtual machine is powered off or VMware Tools is not available, the
Quiesce parameter is not available. You cannot quiesce virtual machines that
have large capacity disks.
IMPORTANT Do not use snapshots as your only backup solution or as a long-term backup solution.
Chapter 16 Managing Virtual Machines
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