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https://issues.apache.org/jira/browse/HDFS-12222?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16108548#comment-16108548
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Kai Zheng commented on HDFS-12222:
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I understand that we need to avoid the confusion between erasure coded and
replicated files when calculating splits for the format. If we want to expose
more info like data blocks and parity blocks (or their locations), one question
is what we can do with them? Note all the blocks or locations need to be used
together to read the data.
> Add EC information to BlockLocation
> -----------------------------------
>
> Key: HDFS-12222
> URL: https://issues.apache.org/jira/browse/HDFS-12222
> Project: Hadoop HDFS
> Issue Type: Bug
> Affects Versions: 3.0.0-alpha1
> Reporter: Andrew Wang
> Labels: hdfs-ec-3.0-nice-to-have
>
> HDFS applications query block location information to compute splits. One
> example of this is FileInputFormat:
> https://github.com/apache/hadoop/blob/d4015f8628dd973c7433639451a9acc3e741d2a2/hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-core/src/main/java/org/apache/hadoop/mapred/FileInputFormat.java#L346
> You see bits of code like this that calculate offsets as follows:
> {noformat}
> long bytesInThisBlock = blkLocations[startIndex].getOffset() +
> blkLocations[startIndex].getLength() - offset;
> {noformat}
> EC confuses this since the block locations include parity block locations as
> well, which are not part of the logical file length. This messes up the
> offset calculation and thus topology/caching information too.
> Applications can figure out what's a parity block by reading the EC policy
> and then parsing the schema, but it'd be a lot better if we exposed this more
> generically in BlockLocation instead.
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