During the VM copy process, Veeam Backup & Replication performs a CRC check for the VM traffic going between the source and target WAN accelerators. The CRC check helps ensure that the correct VM data goes to the target side and no corrupted data blocks are written to the global cache or to backup files in the target backup repository.
The check is performed in the following way:
- Before sending a data block to the target side, Veeam Backup & Replication calculates a checksum for the copied data block.
- Once the data block is copied over WAN and before it is written to the global cache or to the target backup repository, Veeam Backup & Replication re-calculates the checksum for this data block on the target side.
- The source and target checksums are compared. If the checksums do not coincide, the target WAN accelerator sends a request to the source WAN accelerator for the correct data block. The source WAN accelerator re-sends the necessary data blocks to the target WAN accelerator as is and the re-sent data block is written to the global cache or to the backup file on the target backup repository on the fly.