Kihwal Lee created HDFS-8722:
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Summary: 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
Priority: Critical
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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