[
https://issues.apache.org/jira/browse/HDFS-13056?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16392176#comment-16392176
]
Dennis Huo commented on HDFS-13056:
-----------------------------------
Variable-sized blocks within a file are indeed supported (though in your
examples there are still uniform blocksizes within a file except for last
file); in addition to your three examples, the following would also return the
same value as another example:
# [1, 3, 2, 1]
This is implemented by FileChecksumHelper always using block.getBlockSize() as
the data length for each block rather than assuming what the block size should
be, except for the last block, where I don't trust block.getBlockSize(). In the
last block, it turned out that some flows modified the reported block-size
in-place and others didn't so I just made sure to manually compute the correct
number of bytes associated with the last block.
There are constraints on chunk and cell sizes, but these are based on inherent
constraints in the system rather than assumptions in my algorithm; cell size
must be uniform within a block group (but can vary in different block groups
within a single file), which is already a fundamental assumption in striped
reader anyways, and chunk size must be uniform within a block (which is also
fundamental because block metadata doesn't have any way to indicate changes of
bytes-per-crc midway through the block).
> 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
--
This message was sent by Atlassian JIRA
(v7.6.3#76005)
---------------------------------------------------------------------
To unsubscribe, e-mail: hdfs-issues-unsubscr...@hadoop.apache.org
For additional commands, e-mail: hdfs-issues-h...@hadoop.apache.org
