HDFS vs GFS

Block Location State

　　The statement above for HDFS is false.

      The GFS paper, 2.6.2 chunk locations: The master does not store chunck locations persistently.

　　Why? It was much simpler to request chunk location information from chunkservers at startup, and periodically thereafter.

• The problem of keeping the master and chunkservers in sync as chunkservers join and leave the cluster, change names, fail, restart, and so on.
• A chunkserver has the final word over what chunks it does or does not have on its own disks. 

Data Integrity 

　　Data corruption can occur because of faults in a storage device, network faults, or buggy software.

      The statement above about HDFS is not accurate.

　　HDFS

• Checksum unit: a chunk (io.bytes.per.checksum, 512 bytes) 
• End-to-end checksumming (checksums are generated at clients)
• HDFS clients compute checksums and store checksums in a separate hidden file on datanodes
• At rest, DataNodes continuously verify data against stored CRCs to detect and repair bit-rot. (scan period is 504 hrs in HDFS 3.1.2)

　　GFS

• Checksum unit: 64KB sized blocks (chunks are broken up into logical blocks)
• Checksums are generated at chunchsevers
• Each chunkserver must independently verify the integrity of its own copy by maintaining checksums.
• Checksums are kept in memory and stored persistently with logging, separate from user data.
• During idle periods, chunkservers scan and verify the contents of inactive chunks.

　　Note: 1. Read / write unit in GFS may be less than a checksumming block

              2. Replicas of a chunk are not guaranteed to be bitwise identical in GFS

Write Consistency Guarantees

　　consistent

　　　　all clients see the same data,  regardless of which replicas they read from.

　　defined

　　　　A file region is considered 'defined' after a file data mutation if:

• • The file region is consistent
  • clients will see what the mutation writes in its entirety.　　　　

　　The relaxed consistency model in GFS

　　

• Successful Concurrent Writes
  • Write means random write!
  • P1: w(offset_start: 0, offset_end: 100); P2: w(offset_start: 50, offset_end: 150)
• Record Appends
  • Successfully written records are defined
  • There may be paddings or duplicates between defined regions

　　HDFS only allows a single writer which performs serial writes

References

　　Shvachko, Konstantin, et al. "The hadoop distributed file system." MSST. Vol. 10. 2010. paper

　　Ghemawat, Sanjay, Howard Gobioff, and Shun-Tak Leung. "The Google file system." (2003).