When you specify an immutability period for the recent backups, Veeam Agent will automatically add 10 days to the immutability expiration date. This period is called block generation. The block generation period serves to reduce the number of requests to the object storage repository, which results in lower traffic and reduced storage costs. You do not have to configure it, the block generation period is applied automatically.
When the block generation period is appended to the user-defined immutability period, it means there is no need to extend the immutability period for old data blocks when adding new data blocks to the backup during that block generation period.
Consider this example. When you create a full backup to start a backup chain, all data blocks transferred to the object storage repository are new. For these new blocks of data, Veeam Agent will add the block generation period of 10 days to the specified immutability period. If the immutability period is set by user to the default period of 30 days, the effective immutability period with the added block generation period will become 40 days. The first full backup starts its generation that will last for 10 days. All new and reused data blocks within this block generation period will have the same immutability expiration date. For instance, a data block that was transferred to the target repository on day 9 will have the same immutability expiration date as a data block transferred on day 1. This mechanism guarantees that the effective immutability period for all the data blocks within a generation is no less than 30 days.
If a block generation period is over but data blocks from that generation are reused in the newly created checkpoint, their effective immutability period is automatically extended to ensure that the effective immutability period for all the data blocks in the new checkpoint is no less than the user-defined immutability period. For more information, see How Backup Immutability Works.