Ensuring Application Consistency – Checkpoints

This section describes the different options available to ensure application consistency:

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Adding a Checkpoint to a VPG to Identify a Key Point

Checkpoints are recorded automatically every few seconds in the journal. These checkpoints ensure crash-consistency, and are written to the virtual machines journals by the Zerto Virtual Manager.

Each checkpoint has the same timestamp which is set by the Zerto Virtual Manager.

During recovery you pick a checkpoint in the journal and recover to this point. The crash-consistent checkpoints guarantee write order fidelity.

For Example:

If write A on a virtual machine in the VPG occurred before write B on a virtual machine in the VPG, then when a checkpoint is written, the journal will contain:

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Neither of the writes

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Both writes, and if they overlap the B data takes precedence

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Only A, indicating the checkpoint occurred between A and B

The coordination is done by the Zerto Virtual Manager.

You can also use a script to place the application in a quiesced mode, such as Oracle Hot Backup mode, and execute the ZertoPowerShell cmdlet Set-Checkpoint, then release the quiesced mode. For more information about Zerto PowerShell cmdlets, see Zerto Cmdlets.

Note:

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To write application-consistent checkpoints, there is a performance impact on the virtual machine running the application as a result of the application-consistent mechanism used. This is because the guest operating system and any integrated applications will be quiesced.

This impact on performance may be negligible and does not always happen since not all applications require these checkpoints in order to achieve successful application recovery. Also, Zerto only requires the guest and application to quiesce for a brief moment, just long enough to add a checkpoint.

As previously mentioned, checkpoints are recorded every few seconds in the journal. After a while, the number of checkpoints available from which to choose a recovery point can be in excess of thousands per VPG.

When this threshold is reached, in order to enable efficient management and use of the checkpoints, the number of checkpoints is diluted with respect to time, as follows:

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Within the latest 2 hours: All of the checkpoints are available for recovery.

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Between 2 and ~4.5 hours: There are about two to three checkpoints every 15 minutes.

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From 4.5 hours and over: 1 checkpoint is kept every 15 minutes.

Note:

Checkpoints which are either added manually, or marked as part of a Failover test are not diluted.