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https://issues.apache.org/jira/browse/HDFS-3023?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13217895#comment-13217895
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Todd Lipcon commented on HDFS-3023:
-----------------------------------
Apparently I am bad at counting. The above should read "in *four* ways" :)
I ran a teragen with 580 mappers and a 4MB block size on the HA branch with
optimizations 1 and 2 in place and it reduced the size of the edit log from
1.2GB to ~600MB (factor of two savings). This benchmark is designed to log a
lot of persistBlocks() calls, so the savings in less strenuous use cases won't
be as large, but still worth doing.
> Optimize entries in edits log for persistBlocks calls
> -----------------------------------------------------
>
> Key: HDFS-3023
> URL: https://issues.apache.org/jira/browse/HDFS-3023
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: name-node, performance
> Affects Versions: HA branch (HDFS-1623), 0.23.2
> Reporter: Todd Lipcon
> Assignee: Todd Lipcon
>
> One of the performance issues noticed in the HA branch is due to the much
> larger edit logs, now that we are writing OP_ADD transactions to the edit log
> on every block allocation. We can condense these calls down in two ways:
> 1) use variable-length integers for the block list length, size, and genstamp
> (most of these end up fitting in far less than 8 bytes)
> 2) use delta-coding for the genstamp and block size for any blocks after the
> first block (most blocks will be the same size and only slightly higher
> genstamps)
> 3) introduce a new OP_UPDATE_BLOCKS transaction that doesn't re-serialize
> metadata information like lease owner, permissions, etc
> 4) allow OP_UPDATE_BLOCKS to only re-serialize the blocks that have changed
> for a given transaction
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