[jira] Commented: (HADOOP-1470) Rework FSInputChecker and FSOutputSummer to support checksum code sharing between ChecksumFileSystem and block level crc dfs

["Hairong Kuang (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HADOOP-1470?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12509644 ] 

Hairong Kuang commented on HADOOP-1470:
---------------------------------------

Even with  two cascaded buffers, the code seems quite complex. I feel that we should adress the comlexity by breaking it into two classes:
1. BufferedFSInputStream: Provide a buffered input stream and readChunk method
2. FSInputChecker: contains a buffered input stream, align operations on chunk boundaries, and verify checksums

Since this jira is not a regression, I would like to submit a patch without the buffering optimization and then address the optimization in the next release. I'd like to give Raghu sometime to adapt his block-level crc patch to this framework.  Does it work?

> 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, genericChecksum.patch, GenericChecksum1.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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