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https://issues.apache.org/jira/browse/HDFS-13056?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16416732#comment-16416732
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Dennis Huo commented on HDFS-13056:
-----------------------------------
Thanks for taking another look [~xiaochen]! Applied fixes in
[^HDFS-13056.013.patch]. I misinterpreted some of your previous comments
(didn't notice the mention of LimitedPrivate vs Private) which was why I
removed the InterfaceStability annotations; fixed now with
InterfaceStability.Unstable in the ones with LimitedPrivate, and left the
InterfaceAudience.Private ones with no InterfaceStability annotation to let
those default to Unstable.
I went ahead and removed the isDebugEnabled checks in FileChecksumHelper. In
BlockChecksumHelper, I had kept the isDebugEnabled() checks whenever the format
string included calling something like CrcUtil.toMultiCrcString which is
potentially moderately expensive (not too much, but in keeping with where other
places in the code avoid "slightly" expensive things with isDebugEnabled());
prior to [this set of
edits|https://github.com/apache/hadoop/pull/344/commits/391230bf2932a05da76c4a11eadea97b04de4bad#diff-c8fa735a74e1474b5d455843f850b168]
I had used wrapper "Object" that lazy-evaluates toString() to avoid
isDebugEnabled() in favor of just always passing it in as the arg to the log
formatter. I guess the options are:
# Keep the calls to CrcUtil.to*CrcString wrapped inside isDebugEnabled() as-is
# Remove the check and just always perform that string-creation even if it
won't be used by the logger
# Split out a "String crcDebugString = null;" and then assign it to
CrcUtil.toMultiCrcString inside an isDebugEnabled() block, and then
unconditionally pass the crcDebugString into the log format args (this was done
in FileChecksumHelper just because it was convenient for sharing a debug string
between MD5 and CRC codepaths).
I have no strong preference on any of those options.
Added global timeouts of 10 seconds to TestCrcUtil and TestCrcComposer.
> 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, HDFS-13056.009.patch, HDFS-13056.010.patch,
> HDFS-13056.011.patch, HDFS-13056.012.patch, HDFS-13056.013.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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