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https://issues.apache.org/jira/browse/HADOOP-1470?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12506279
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Raghu Angadi commented on HADOOP-1470:
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Chatted with Sameer regd preads in DFS, and this is what he has in mind :
DFSInputStream() opens on InputChecker for each block it is reading from. So
normally there would be one 'blockStream'. In case of pread, instead of
creating a new DFSInputStream as I was thinking, it would create one
InputChecker with each block involved (in sequence, it could even fetch data
parallely in future). So pread() will not create a new DFSInputStream(). This
will use all the Block cache etc. This looks good to me. The current Block
Level CRCs has similar structure (it creates one {{BlockStream}} object for
each block etc).
Also above pread implementation is idependent of actual implementation of
InputChecker. ie it is independent of whether InputChecker uses 'readChunk()',
'readChecksum()' or 'two streams'.
> Rework FSInputChecker and FSOutputSummer to support checksum code sharing
> between ChecksumFileSystem and block level crc dfs
> ----------------------------------------------------------------------------------------------------------------------------
>
> Key: HADOOP-1470
> URL: https://issues.apache.org/jira/browse/HADOOP-1470
> Project: Hadoop
> Issue Type: Improvement
> Components: fs
> Affects Versions: 0.12.3
> Reporter: Hairong Kuang
> Assignee: Hairong Kuang
> Fix For: 0.14.0
>
> Attachments: genericChecksum.patch, InputChecker-01.java
>
>
> Comment from Doug in HADOOP-1134:
> I'd prefer it if the CRC code could be shared with CheckSumFileSystem. In
> particular, it seems to me that FSInputChecker and FSOutputSummer could be
> extended to support pluggable sources and sinks for checksums, respectively,
> and DFSDataInputStream and DFSDataOutputStream could use these. Advantages of
> this are: (a) single implementation of checksum logic to debug and maintain;
> (b) keeps checksumming as close to possible to data generation and use. This
> patch computes checksums after data has been buffered, and validates them
> before it is buffered. We sometimes use large buffers and would like to guard
> against in-memory errors. The current checksum code catches a lot of such
> errors. So we should compute checksums after minimal buffering (just
> bytesPerChecksum, ideally) and validate them at the last possible moment
> (e.g., through the use of a small final buffer with a larger buffer behind
> it). I do not think this will significantly affect performance, and data
> integrity is a high priority.
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