FileChecksum was first introduced in https://issues-test.apache.org/jira/browse/HADOOP-3981 and ever since then has remained defined as MD5-of-MD5-of-CRC, where per-512-byte chunk CRCs are already stored as part of datanode metadata, and the MD5 approach is used to compute an aggregate value in a distributed manner, with individual datanodes computing the MD5-of-CRCs per-block in parallel, and the HDFS client computing the second-level MD5.
A shortcoming of this approach which is often brought up is the fact that this FileChecksum is sensitive to the internal block-size and chunk-size configuration, and thus different HDFS files with different block/chunk settings cannot be compared. More commonly, one might have different HDFS clusters which use different block sizes, in which case any data migration won't be able to use the FileChecksum for distcp's rsync functionality or for verifying end-to-end data integrity (on top of low-level data integrity checks applied at data transfer time).
This was also revisited in https://issues.apache.org/jira/browse/HDFS-8430 during the addition of checksum support for striped erasure-coded files; while there was some discussion of using CRC composability, it still ultimately settled on hierarchical MD5 approach, which also adds the problem that checksums of basic replicated files are not comparable to striped files.
This feature proposes to add a "COMPOSITE-CRC" FileChecksum type which uses CRC composition to remain completely chunk/block agnostic, and allows comparison between striped vs replicated files, between different HDFS instances, and possible even between HDFS and other external storage systems. This feature can also be added in-place to be compatible with existing block metadata, and doesn't need to change the normal path of chunk verification, so is minimally invasive. This also means even large preexisting HDFS deployments could adopt this feature to retroactively sync data. A detailed design document can be found here: https://storage.googleapis.com/dennishuo/hdfs-file-composite-crc32-v1.pdf
|Add support for new COMPOSITE_CRC in WebHDFS||Open||Unassigned|