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https://issues.apache.org/jira/browse/HDFS-17463?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17837542#comment-17837542
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ASF GitHub Bot commented on HDFS-17463:
---------------------------------------
XbaoWu opened a new pull request, #6736:
URL: https://github.com/apache/hadoop/pull/6736
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### Description of PR
Hadoop 3.2 introduced optimization features for HDFS StringTable
(b60ca37914b22550e3630fa02742d40697decb3), It resulted in lower versions of
Hadoop upgraded to 3.2 and later versions not supporting downgrade operations.
Therefore, we have added an optimization to support downgrading. In the
upgrade with large version span, it avoids the low version can not read the
fs-image generated by the 3.2 and later version, which support for the 3.2 and
later version to be downgraded to the low version.
For more information about this PR, please refer to the following issue：
[HDFS-17463](https://issues.apache.org/jira/browse/HDFS-17463) Support the
switch StringTable Split ID feature
### How was this patch tested?
### For code changes:
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(e.g. 'HADOOP-17799. Your PR title ...')?
- [ ] Object storage: have the integration tests been executed and the
endpoint declared according to the connector-specific documentation?
- [ ] If adding new dependencies to the code, are these dependencies
licensed in a way that is compatible for inclusion under [ASF
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`NOTICE-binary` files?
> Support the switch StringTable Split ID feature
> -----------------------------------------------
>
> Key: HDFS-17463
> URL: https://issues.apache.org/jira/browse/HDFS-17463
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: namenode
> Affects Versions: 3.2.0, 3.3.5, 3.3.3, 3.3.4
> Reporter: wangzhihui
> Priority: Major
> Attachments: Image_struct.png, error.png
>
>
> desc:
> *
> Hadoop 3.2 introduced optimization features for HDFS StringTable
> (b60ca37914b22550e3630fa02742d40697decb3), It resulted in lower versions of
> Hadoop upgraded to 3.2 and later versions not supporting downgrade
> operations.
> !error.png!
> * This issue has also been discussed in HDFS-14831, and it is recommended to
> revert the feature, but it cannot fundamentally solve the problem。
> *
> Therefore, we have added an optimization to support downgrading
>
> Solution:
> * First, we will add the "dfs. image. save. splitId. stringTable" conf
> switch "StringTable optimization feature" is enabled
> * When the conf value is false, an Image file compatible with lower versions
> of HDFS is generated to support downgrading.
> *
> The difference in HDFS Image file format between Hadoop 3.1.1 and Hadoop 3.2
> is shown in the following figure.
> * With the sub-sections feature introduced in HDFS-14617, Protobuf can
> support compatible reading.
> *
> The data structure causing incompatible differences is mainly StringTable.
> !Image_struct.png|width=396,height=163!
> * In "dfs.image.save.splitId.stringTable = false " the Id generation order
> of StringTable starts from 0 to Integer.Max. When true, the Id value range
> follows the latest rules.
>
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