Application-consistent backups are used to back up entities with high or extremely high transaction loads. Good examples of such entities can be Microsoft SharePoint, Active Directory, MS SQL Server, or Oracle database solutions.
When such high-transactional solutions become subject to back up, the key point is the memory state and current transactions are stopped, flushed to disk, and only then are backed up.
If memory cannot be flushed to disk for some reason, backup may not be valid for restore or some important data may be missing.
Thus, an application-consistent backup is a backup that is equipped with tools to notify applications about coming backup operation and allow applications to achieve a quiescent and consistent state by flushing all operations to disk.
- Frameworks for Application-Consistent Backups
- VMware Application-Consistent Backups
- Application-Consistent Hyper-V Backups
Frameworks for Application-Consistent Backups
Volume Shadow Copy Service (VSS) is a framework designed primarily to solve the application-consistent problems for Windows operating systems. VSS is an infrastructure that provides backup applications with the feature of creating application-consistent backups.
When backup job is initiated, VSS works as follows:
- Every running application that needs to flush its data kept in RAM to disk is notified by VSS.
- Then a Copy-On-Write operation is performed on volume.
- A volume snapshot is done and can be processed by a backup application.
In case an application runs on a virtual machine on Hyper-V Server, a VSS-aware backup of the Hyper-V host, leveraging Hyper-V Integration Services, communicates with the VSS writers in the guest virtual machine to perform an application-consistent backup of the virtual machine and the applications on it
VMware Application-Consistent Backups
Application-consistent backup of virtual machines on VMware environment is based on quiescence.
Note that the application-consistent backups are not supported on all guest operating systems. Read the Application-Consistent backups are unsupported in Linux virtual machine's (2044492) article at kb.vmware.com to learn a list of operating systems that support application-consistent backups
Quiescing is a process of bringing virtual machine file systems and application data to a consistent state. This is an important tool for correct application-consistent virtual machine backups.
To back up a currently running VM, quiescence is an efficient method. This concerns a virtual environment with highly-transactional applications (e.g. MS SQL Server or alike) since the safety of the application data is guaranteed by quiescence.
For consistent backup of virtual machines with no VSS support (Linux VMs), a crash-consistent quiescing is offered.
Also, quiescence suits to back up Microsoft Windows-based VMs with the built-in Microsoft VSS support. For these purposes, the VMware VSS component of VMware Tools is used.
To learn more, refer to Working with VSS chapter of the VMware documentation
Configure VMware Application-Consistent Backups
You can configure the application-consistent settings on the Select Virtual Machines step of the VMware backup wizard.
Click Advanced Settings.
The following options are available:
- Back up virtual machines with application-consistent issues. Once this option is selected, virtual machines are backed up one by one. With VMware Tools, the state of applications running on virtual machines is checked, then a snapshot is made and an application-consistent backup is performed. In case an application-consistent snapshot is not created for some reason, a regular snapshot of this virtual machine is performed
- Skip virtual machines with application-consistent issues. Once this option is selected, virtual machines with applications that did not flush pending I/O operations from memory to disks, are skipped, and an appropriate warning is displayed
- Do not use application-consistent backup. Once this option is selected, regular VM snapshots are created without quiescing
Application-Consistent Hyper-V Backups
Application-consistent backups of virtual machines in a Hyper-V environment are based on interactive checkpoint type selection.
Available Hyper-V Checkpoint Types
Windows 10 Hyper-V includes two types of checkpoints:
- Standard Checkpoints. A standard checkpoint takes a snapshot of the virtual machine and virtual machine memory state at the time the checkpoint is initiated. A snapshot is not a full backup and can cause data consistency issues with systems that replicate data between different nodes such as Active Directory. Hyper-V only offered standard checkpoints (formerly called snapshots) prior to Windows 10
- Production Checkpoints. Production checkpoint uses Volume Shadow Copy Service or File System Freeze on a Linux virtual machine to create a data-consistent backup of the virtual machine. No snapshot of the virtual machine memory state is taken.
Checkpoint creation logic in CloudBerry Backup depends on Hyper-V configuration. The following cases are possible:
- Production checkpoints are selected
- Standard checkpoints are selected
- Checkpoints are disabled (no checkpoint type selected)
Backup for Windows always starts by trying to create a checkpoint of the selected type first. In the event of a failure, a recovery inconsistent checkpoint will be created. The recovery inconsistent checkpoint does not contain the VM VSS snapshot. If no checkpoint type is set, MSP360 (CloudBerry) Backup starts by trying to create a recovery consistent checkpoint using VM VSS snapshot. In the event of a failure, a recovery inconsistent checkpoint will be created. The recovery inconsistent checkpoint does not contain the VM VSS snapshot. In case of recovery consistent checkpoint usage, I/O activities that are quiesced at a point of creating VM VSS snapshot will be resumed to ensure successful restore. In case of the recovery inconsistent checkpoint is used for restore, the restore may fail.
To learn how to configure checkpoints in your Hyper-V environment, refer to Using checkpoints to revert virtual machines to a previous state chapter at docs.microsoft.com