0.94 hfiles v2.1 are not backwards compatible with HFilev2.0

When doing a rolling re-start from 0.92.x to 0.94.x any hfiles written by 0.94 are incompatibile with any of the 0.92 region servers. This is caused by the checksums being put into 0.94.

• a minor version was added
• checksums were put into the block
• checksum meta data was added to block headers.

I propose that since these changes are only needed if using hbase.regionserver.checksum.verify, they should be turned off if that option is turned off. Doing so will allow rolling upgrades to go smoother.

If a user wants to go from a 0.92 cluster to a 0.94 cluster with hbase.regionserver.checksum.verify they can:

• Roll out 0.94
• Change hbase-site.xml
• roll restart the region servers.