If the protected object to be replicated is a running virtual machine without VMTools installed, the replication service of Arcfra Backup & Disaster Recovery creates a crash-consistent snapshot for the protected object. This type of snapshot records only disk data and does not force cached data to be flushed to disk. Therefore, if some cached data of the protected object has not been fully written to disk, the created snapshot may have inconsistencies between cached data and disk data, which may cause some files to become corrupted after the protected object is restored.
If the protected object to be replicated is a running virtual machine with VMTools installed, the replication service uses VMTools to create a file-system-consistent virtual machine snapshot during replication, ensuring consistency between cached data and disk data so that the virtual machine can work properly after failover. Therefore, you are advised to install VMTools for virtual machines to be asynchronously replicated.
In different operating systems, VMTools creates file-system-consistent snapshots based on the following principles:
Linux virtual machine
Linux uses FIFREEZE to freeze the file system. It first blocks write operations on the file system, and then syncs the inode information and dirty pages of the file system. After the file system is frozen, ABS APIs are called to create a volume snapshot. After the snapshot is created, the file system is unfrozen.
Windows virtual machine
Windows uses Volume Shadow Copy Service (VSS) to create file-system-consistent snapshots. For implementation details, see the official Windows documentation.
