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| JIRA Issue | HDFS-13056 |
| JIRA Patch URL | 
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> Expose file-level composite CRCs in HDFS which are comparable across 
> different instances/layouts
> ------------------------------------------------------------------------------------------------
>                 Key: HDFS-13056
>                 URL: https://issues.apache.org/jira/browse/HDFS-13056
>             Project: Hadoop HDFS
>          Issue Type: New Feature
>          Components: datanode, distcp, erasure-coding, federation, hdfs
>    Affects Versions: 3.0.0
>            Reporter: Dennis Huo
>            Priority: Major
>         Attachments: HDFS-13056-branch-2.8.001.patch, 
> HDFS-13056-branch-2.8.002.patch, HDFS-13056-branch-2.8.poc1.patch, 
> HDFS-13056.001.patch, HDFS-13056.002.patch, HDFS-13056.003.patch, 
> HDFS-13056.003.patch, HDFS-13056.004.patch, 
> Reference_only_zhen_PPOC_hadoop2.6.X.diff, hdfs-file-composite-crc32-v1.pdf, 
> hdfs-file-composite-crc32-v2.pdf, hdfs-file-composite-crc32-v3.pdf
> 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

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