[HDFS-4403] DFSClient can infer checksum type when not provided by reading first byte

• Type: Bug
• Status: Closed
• Priority: Minor
• Resolution: Fixed
• Affects Version/s: 2.0.2-alpha
• Fix Version/s: 2.0.3-alpha
• Component/s: hdfs-client
• Target Version/s: 2.0.3-alpha
• Hadoop Flags: Reviewed

Release Note

The HDFS implementation of getFileChecksum() can now operate correctly against earlier-version datanodes which do not include the checksum type information in their checksum response. The checksum type is automatically inferred by issuing a read of the first byte of each block.

• Assignee: Todd Lipcon
• Reporter: Todd Lipcon
• Created: 15/Jan/13 01:11
• Updated: 03/Sep/14 23:09
• Resolved: 22/Jan/13 02:59

Description

HDFS-3177 added the checksum type to OpBlockChecksumResponseProto, but the new protobuf field is optional, with a default of CRC32. This means that this API, when used against an older cluster (like earlier 0.23 releases) will falsely return CRC32 even if that cluster has written files with CRC32C. This can cause issues for distcp, for example.

Instead of defaulting the protobuf field to CRC32, we can leave it with no default, and if the OpBlockChecksumResponseProto has no checksum type set, the client can send OP_READ_BLOCK to read the first byte of the block, then grab the checksum type out of that response (which has always been present)