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https://issues.apache.org/jira/browse/HDFS-6596?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14042986#comment-14042986
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Zesheng Wu commented on HDFS-6596:
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Thanks Colin.
bq. What you are proposing is basically making every {{read}} into a
{{readFully}}. I don't think we want to increase the number of differences
between how DFSInputStream works and how "normal" Java input streams work. The
"normal" java behavior also has a good reason behind it... clients who can deal
with partial reads will get a faster response time if the stream just returns
what it can rather than waiting for everything. In the case of HDFS, waiting
for everything might mean connecting to a remote DataNode. This could be quite
a lot of latency.
I agree with you that we shouldn't make every {{read}} into a {{readFully}},
and the current implementation of {{read}} has its advantage as you described.
About the solution, I think that we do it in Hadoop will be better, because all
users will be benefited.
The current {{readFully}} for DFSInputStream is implemented as pread and
inherits from FSInputStream, so I will a new {{readFully(buffer, offset,
length)}} to figure this out. Any thoughts?
> Improve InputStream when read spans two blocks
> ----------------------------------------------
>
> Key: HDFS-6596
> URL: https://issues.apache.org/jira/browse/HDFS-6596
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: hdfs-client
> Affects Versions: 2.4.0
> Reporter: Zesheng Wu
> Assignee: Zesheng Wu
>
> In the current implementation of DFSInputStream, read(buffer, offset, length)
> is implemented as following:
> {code}
> int realLen = (int) Math.min(len, (blockEnd - pos + 1L));
> if (locatedBlocks.isLastBlockComplete()) {
> realLen = (int) Math.min(realLen, locatedBlocks.getFileLength());
> }
> int result = readBuffer(strategy, off, realLen, corruptedBlockMap);
> {code}
> From the above code, we can conclude that the read will return at most
> (blockEnd - pos + 1) bytes. As a result, when read spans two blocks, the
> caller must call read() second time to complete the request, and must wait
> second time to acquire the DFSInputStream lock(read() is synchronized for
> DFSInputStream). For latency sensitive applications, such as hbase, this will
> result in latency pain point when they under massive race conditions. So here
> we propose that we should loop internally in read() to do best effort read.
> In the current implementation of pread(read(position, buffer, offset,
> lenght)), it does loop internally to do best effort read. So we can refactor
> to support this on normal read.
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