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https://issues.apache.org/jira/browse/HDFS-17970?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18109589#comment-18109589
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Jay Zou edited comment on HDFS-17970 at 8/31/26 12:51 AM:
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I am working on a fix and will submit a pull request with a regression test.
was (Author: JIRAUSER292203):
{{I am working on a fix and will submit a pull request with a regression test.}}
> EC checksum reconstruction should exclude the failed checksum target from
> source inputs
> ---------------------------------------------------------------------------------------
>
> Key: HDFS-17970
> URL: https://issues.apache.org/jira/browse/HDFS-17970
> Project: Hadoop HDFS
> Issue Type: Bug
> Components: datanode, ec, erasure-coding
> Affects Versions: 3.2.2, 3.5.0
> Environment: Apache Hadoop trunk base:
> 33237971fa49f7ba0a0f4e2790dfdd7b792e8acc
> Operating system: macOS 15.6
> Java: Eclipse Temurin 17.0.6
> Test environment: MiniDFSCluster
> Erasure coding policy: default RS-6-3 policy
> dfs.datanode.ec.reconstruction.validation=true
> The same source construction is present in the Hadoop 3.5.0 release and
> current upstream trunk.
> Reporter: Jay Zou
> Priority: Major
> Labels: pull-request-available
>
> h3. Problem
> When a child BLOCK_CHECKSUM request fails for an internal block while the
> block remains available for READ_BLOCK,
> BlockGroupNonStripedChecksumComputer#recalculateChecksum() marks the failed
> block index as a reconstruction target, but still passes the original
> blockIndices and datanodes arrays to StripedReconstructionInfo.
> The same internal block can therefore be selected as both a reconstruction
> target and a source. This violates the decoder input contract, which requires
> the input corresponding to an erased index to be null.
> h3. Impact
> With dfs.datanode.ec.reconstruction.validation=true, validation can be left
> with fewer than the required number of independent inputs and fail with:
> {code}
> No enough valid inputs are provided, not recoverable
> {code}
> The client may subsequently observe Unexpected EOF.
> When reconstruction validation is disabled, the decoder may read the target
> block to reconstruct the same target. The resulting checksum may happen to
> match, but the reconstruction input still violates the source/target
> disjointness invariant.
> h3. Reproduction
> A deterministic MiniDFSCluster test reproduces the problem as follows:
> # Enable dfs.datanode.ec.reconstruction.validation.
> # Create an erasure-coded file.
> # Obtain a normal BLOCK_GROUP_CHECKSUM response as the baseline.
> # Make one child BLOCK_CHECKSUM request fail by replacing only its child
> block token.
> # Keep the target DataNode and internal block online and readable.
> # Request BLOCK_GROUP_CHECKSUM again.
> The reconstruction READ_BLOCK path obtains a fresh read token, so the target
> remains readable after its child checksum request fails.
> Before the fix, both checksum-mode test instances fail through:
> {code}
> InvalidBlockTokenException
> -> recalculateChecksum()
> -> DecodingValidator.validate()
> -> No enough valid inputs are provided, not recoverable
> -> Unexpected EOF
> {code}
> h3. Expected behavior
> Checksum reconstruction should:
> * exclude every source position whose internal block index matches the target;
> * preserve block-index and DataNode pairing;
> * place distinct source indices before duplicate replicas;
> * retain non-target duplicate replicas as read fallbacks;
> * use the short-block-group-aware minimum source count;
> * fail with a contextual IOException when there are not enough distinct
> sources.
> h3. Related issues
> HDFS-15650 makes the EC checksum socket timeout configurable, and HDFS-17806
> discusses increasing that timeout. They can reduce how often reconstruction is
> triggered but do not enforce source/target disjointness.
> HDFS-15759 introduced DataNode EC reconstruction validation, and HDFS-15795
> fixed checksum handling after reconstruction exceptions. Neither excludes the
> failed checksum target from reconstruction sources.
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