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https://issues.apache.org/jira/browse/HDFS-13056?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16392107#comment-16392107
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Arpit Agarwal commented on HDFS-13056:
--------------------------------------

Hi [~dennishuo], I haven't read your design doc in detail, but I have a 
question about variable length blocks.

Supposing a 7 byte file is stored in multiple different schemes:
# Single 7 byte block -> [7]
# As two blocks -> [4, 3]
# As 7 different blocks -> [1, 1, 1, 1, 1, 1, 1]

Does this scheme ensure that the composite CRC will be the same in all cases? 
Or is there an assumption that all but the last block will be the same size?

> 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
>            Assignee: Dennis Huo
>            Priority: Major
>         Attachments: HDFS-13056-branch-2.8.001.patch, 
> HDFS-13056-branch-2.8.002.patch, HDFS-13056-branch-2.8.003.patch, 
> HDFS-13056-branch-2.8.004.patch, HDFS-13056-branch-2.8.005.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, 
> HDFS-13056.005.patch, HDFS-13056.006.patch, HDFS-13056.007.patch, 
> HDFS-13056.008.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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