How mysqlcluster can be durable and consistent?

[urgen sherpa]

I'm little noob and learning on how to use mysqlcluster,i have 1 management 2 datanode and 2 sqlnodes.According to a book "mysql cookbook" -alex davies and http://dev.mysql.com/doc/ndbapi/en/ndb-internals-ndbd-filesystemdir-files.html

By default, a running MySQL Cluster will commit a Local Checkpoint (LCP)—a copyof all the local data held on disk—every time 4 MB of changes are made to the cluster (since
the previous LCP) and global checkpoint (all transactions that have occurred since the last LCP) to disk every two seconds. if the data are fragmented and parts of large tables are striped on those datanodes(database "A" is 1000G and may be divided into two 500G parts) ,

`Q1) How can a database still consistent and durable if any one of the datanode server is crashed ?`

With Reference to https://dev.mysql.com/doc/refman/5.7/en/mysql-cluster-programs-ndb-mgmd.html if i take backup
using "start backup" on management node, it will create backup on the backup directory on each Data Nodes but each backup will only hold the different fragment of table data instead of complete backup(backup taken by mysqldump). I tested backup and it was true where complete database size was around 44G and the backupdir was around 22G on each datanode backup directory. I'am confused and please correct me if I have wrong misconception