[ https://issues.apache.org/jira/browse/HDFS-13056?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]
Dennis Huo updated HDFS-13056: ------------------------------ Status: Open (was: Patch Available) > 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, > 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