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https://issues.apache.org/jira/browse/HBASE-16891?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15614945#comment-15614945
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Duo Zhang commented on HBASE-16891:
-----------------------------------
I'm not sure but I think it will change the bb's position. But it does not
matter since I will call duplicate in the upper layer. As you said, ByteBuf
does not support write(bb, off, len).
{code}
@Override
public void write(ByteBuffer b, int off, int len) throws IOException {
ByteBuffer bb = b.duplicate();
bb.position(off);
bb.limit(off + len);
out.write(bb);
}
{code}
> Try copying to the Netty ByteBuf directly from the WALEdit
> ----------------------------------------------------------
>
> Key: HBASE-16891
> URL: https://issues.apache.org/jira/browse/HBASE-16891
> Project: HBase
> Issue Type: Sub-task
> Components: wal
> Affects Versions: 2.0.0
> Reporter: ramkrishna.s.vasudevan
> Assignee: ramkrishna.s.vasudevan
> Fix For: 2.0.0
>
> Attachments: HBASE-16891-v1.patch, HBASE-16891-v2.patch,
> HBASE-16891.patch
>
>
> -> The FanOutOneBlockAsyncDFSOutput is a much sophisticated dfs client model
> that works with Netty ByteBuf. Here we hold on connection to the datanodes
> using
> Netty Channels. And the idea is to write data direclty to these channels.
> AsyncHLog gets an append call. The AysncWAL uses the HBase's
> ByteArrayOutputSTream and so the content of the cell is written to this BAOS
> and that is again
> copied to the netty Bytebuf in the FanOutOneBlockAsyncDFSOutput.
> So when the sync call happens this FanoutDFSoutput does the checksum
> calcualtion itself and then writes the content of this buffer direclty to the
> DN channel.
> -> In case of FSHLOg this is different. When an append call comes we direclty
> write the content to the FSDataOutputStream (it is copied to this stream).
> Then here internally there is a checkSum calculation that happens. when a
> sync call happens there is noth ing to do except to notify the NN to flush
> the latest
> data.
> AS we can see from the above that there are two copies in AsyncWAL
> -> From the Cell to the BAOS
> -> From the BAOS to the Netty byte buf
> -> On sync() call, do check sum and finally flush the netty byte buf to the
> DN channel
> In case of FSHLog
> -> From cell to the FSDataoutputstream. data is copied. Check sum happens
> here.
> -> Sync call just tries to notify the NN.
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