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https://issues.apache.org/jira/browse/HBASE-21879?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16917309#comment-16917309
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Duo Zhang commented on HBASE-21879:
-----------------------------------

Can we finish this now? [~openinx]

And I do not think we can wait for a patch in HDFS, as we still need to support 
legacy versions of hdfs, we can not make use of the new methods actually, until 
we drop all the legacy versions.

What do you think?

Thanks.

> Read HFile's block to ByteBuffer directly instead of to byte for reducing 
> young gc purpose
> ------------------------------------------------------------------------------------------
>
>                 Key: HBASE-21879
>                 URL: https://issues.apache.org/jira/browse/HBASE-21879
>             Project: HBase
>          Issue Type: Improvement
>            Reporter: Zheng Hu
>            Assignee: Zheng Hu
>            Priority: Major
>             Fix For: 3.0.0, 2.3.0
>
>         Attachments: HBASE-21879.v1.patch, HBASE-21879.v1.patch, 
> QPS-latencies-before-HBASE-21879.png, gc-data-before-HBASE-21879.png
>
>
> In HFileBlock#readBlockDataInternal,  we have the following: 
> {code}
> @VisibleForTesting
> protected HFileBlock readBlockDataInternal(FSDataInputStream is, long offset,
>     long onDiskSizeWithHeaderL, boolean pread, boolean verifyChecksum, 
> boolean updateMetrics)
>  throws IOException {
>  // .....
>   // TODO: Make this ByteBuffer-based. Will make it easier to go to HDFS with 
> BBPool (offheap).
>   byte [] onDiskBlock = new byte[onDiskSizeWithHeader + hdrSize];
>   int nextBlockOnDiskSize = readAtOffset(is, onDiskBlock, preReadHeaderSize,
>       onDiskSizeWithHeader - preReadHeaderSize, true, offset + 
> preReadHeaderSize, pread);
>   if (headerBuf != null) {
>         // ...
>   }
>   // ...
>  }
> {code}
> In the read path,  we still read the block from hfile to on-heap byte[], then 
> copy the on-heap byte[] to offheap bucket cache asynchronously,  and in my  
> 100% get performance test, I also observed some frequent young gc,  The 
> largest memory footprint in the young gen should be the on-heap block byte[].
> In fact, we can read HFile's block to ByteBuffer directly instead of to 
> byte[] for reducing young gc purpose. we did not implement this before, 
> because no ByteBuffer reading interface in the older HDFS client, but 2.7+ 
> has supported this now,  so we can fix this now. I think. 
> Will provide an patch and some perf-comparison for this. 



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