Aure Backup Snapshot Consistency Details

Question

Hi folks,

May I please request you all to help me understand the different Azure Backup consistency types in detail.

Any help is highly appreciated!

Cheers!
Apurva

Answer 1

@Apurva Pathak When you perform a backup of an Azure VM, there are two types of backups that can be performed: application-consistent backups and file system-consistent backups.

Application-consistent backups use the Volume Shadow Copy Service (VSS) to ensure that all data in memory and on disk is in a consistent state at the time of the backup. This is important for applications that have databases or other data that is constantly changing.

File system-consistent backups, on the other hand, do not use VSS and only ensure that the file system is in a consistent state at the time of the backup. This type of backup is typically used for file servers or other systems that do not have databases or other applications that require application-consistent backups.

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Answer 2

Refer to https://learn.microsoft.com/en-us/azure/backup/backup-azure-vms-introduction#snapshot-consistency

Snapshot	Details	Recovery	Consideration
Application-consistent	This is the default setting in the VM backup policy. App-consistent backups capture memory content and pending I/O operations. App-consistent snapshots use a VSS writer (or pre/post scripts for Linux) to ensure the consistency of the app data before a backup occurs.	When you're recovering a VM with an app-consistent snapshot, the VM boots up. There's no data corruption or loss. The apps start in a consistent state.	Windows: All VSS writers succeeded

Linux: Pre/post scripts are configured and succeeded| |Application-consistent|This is the default setting in the VM backup policy. App-consistent backups capture memory content and pending I/O operations. App-consistent snapshots use a VSS writer (or pre/post scripts for Linux) to ensure the consistency of the app data before a backup occurs.|When you're recovering a VM with an app-consistent snapshot, the VM boots up. There's no data corruption or loss. The apps start in a consistent state.|Windows: All VSS writers succeeded Linux: Pre/post scripts are configured and succeeded| |File-system consistent|This is the default setting in the VM backup policy. File-system consistent backups provide consistency by taking a snapshot of all files at the same time.|When you're recovering a VM with a file-system consistent snapshot, the VM boots up. There's no data corruption or loss. Apps need to implement their own "fix-up" mechanism to make sure that restored data is consistent.|Windows: Some VSS writers failed Linux: Default (if pre/post scripts aren't configured or failed)| |Crash-consistent|Crash-consistent snapshot is an opt-in setting in the VM backup policy. Azure Backup also takes crash-consistent backups if the VM is not running during backup and when application/file-consistent backups fail. Only the data that already exists on the disk at the time of the backup operation is captured and backed up; data in read/write host cache isn't captured.|Starts with the VM boot process followed by a disk check to fix corruption errors. Any in-memory data or write operations that weren't transferred to disk before the crash are lost. Apps implement their own data verification. For example, a database app can use its transaction log for verification. If the transaction log has entries that aren't in the database, the database software rolls transactions back until the data is consistent.|When you have opted for application/file-system backup and VM is in shutdown (stopped/ deallocated) state and when the snapshot is retried. You have opted for agentless crash consistent backups|

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hth

Marcin