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From "Doug Cutting (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-1470) Rework FSInputChecker and FSOutputSummer to support checksum code sharing between ChecksumFileSystem and block level crc dfs
Date Tue, 26 Jun 2007 18:25:26 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-1470?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12508272
] 

Doug Cutting commented on HADOOP-1470:
--------------------------------------

It sounds like we're reaching convergence.  The InputChecker abstract methods might be something
like:

{code}
/** Return position of beginning of chunk containing pos. */
long getChunkPosition(long pos);
/** Return the maximum length of chunks in this file. */
int getChunkSize();
/** Return the length of each checksum. */
int getChecksumSize();
/** Read a full chunk and its checksum at the specified position, returning its length. */
int readChunk(long position, byte[] buffer, int start, int length, byte[] checksum);
{code}

Is that sufficient that an input stream can be fully implemented w/o overriding other methods?

Note that I've added the checksum into readChunk() to simplify synchronization and pread.
 Does that look reasonable?


> 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, 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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