Here is a rolling upgrade truncate test from Brandon Li. The basic test step is: (3 nodes cluster with HA)
1. upload a file to hdfs
2. start rollingupgrade. finish rollingupgrade for namenode and one datanode.
3. truncate the file in hdfs to 1byte
4. do rollback
5. download file from hdfs, check file size to be original size
I see the file size in hdfs is correct but can't read it because the block is corrupted.