As discussed on IRC, we do modify xtrabackup_logfile on prepare to 1) update the logfile header with checkpoint info and 2) to expand and align it if necessary, because InnoDB recovery code will bark at redo log if it's smaller than 2 MB.
On the other hand, those modifications can be done right after finishing the backup, so for example, incremental backups could be on a read-only media when merging them to a full one.
As discussed on IRC, we do modify xtrabackup_logfile on prepare to 1) update the logfile header with checkpoint info and 2) to expand and align it if necessary, because InnoDB recovery code will bark at redo log if it's smaller than 2 MB.
On the other hand, those modifications can be done right after finishing the backup, so for example, incremental backups could be on a read-only media when merging them to a full one.