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https://issues.apache.org/jira/browse/HDFS-8722?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14619104#comment-14619104
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Kihwal Lee commented on HDFS-8722:
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Forgot to remove one line in the patch.
> Optimize datanode writes for small writes and flushes
> -----------------------------------------------------
>
> Key: HDFS-8722
> URL: https://issues.apache.org/jira/browse/HDFS-8722
> Project: Hadoop HDFS
> Issue Type: Improvement
> Reporter: Kihwal Lee
> Assignee: Kihwal Lee
> Priority: Critical
> Attachments: HDFS-8722.patch
>
>
> After the data corruption fix by HDFS-4660, the CRC recalculation for partial
> chunk is executed more frequently, if the client repeats writing few bytes
> and calling hflush/hsync. This is because the generic logic forces CRC
> recalculation if on-disk data is not CRC chunk aligned. Prior to HDFS-4660,
> datanode blindly accepted whatever CRC client provided, if the incoming data
> is chunk-aligned. This was the source of the corruption.
> We can still optimize for the most common case where a client is repeatedly
> writing small number of bytes followed by hflush/hsync with no pipeline
> recovery or append, by allowing the previous behavior for this specific case.
> If the incoming data has a duplicate portion and that is at the last
> chunk-boundary before the partial chunk on disk, datanode can use the
> checksum supplied by the client without redoing the checksum on its own.
> This reduces disk reads as well as CPU load for the checksum calculation.
> If the incoming packet data goes back further than the last on-disk chunk
> boundary, datanode will still do a recalculation, but this occurs rarely
> during pipeline recoveries. Thus the optimization for this specific case
> should be sufficient to speed up the vast majority of cases.
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