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From "Raghu Angadi (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, 12 Jun 2007 00:38:26 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-1470?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12503690
] 

Raghu Angadi commented on HADOOP-1470:
--------------------------------------

Let me show what I had in my mind when I proposed "genericChecksum.java" above on Jun 8th:
Main and only aim of it is to share the read and retry loops. readChunk() and readChecksum
are not public interfaces in the sense, map/reduce never invokes it. FSInputStream public
interface does not change.

psuedo code of the class and how it is used. :

{code:title:inputChecker.java}

//First the use case:

FSInputChecker in ChecksumFileSystem would look like this:

class FSInputChecker extends FSInputStream  {

  InputCheker checker = new InputChecker(this);

   public int read(buf, offset, len) {
       return checker.read(buf, offset, len);
   }
   
   implement readChunk();
   implement readChecksum() ;

   seek etc slightly modify.
}



class InputChecker {
 
/// FSInputChecker supports readChunk() and readChecksum() as described in my comment
InputChecker(FSInputChecker in);

ReadInfo readInfo;
FSInputStream in;

// contract is same as inputStrem.read().
// following is too simplified. But real implementation will look a bit more involved but
that meat is shared across the file systems, which I think is what this
// Jira wants.

int read(byte[] userBuf, int offset, int len) {
  int bytesRead = 0
  while ( bytesRead < len )  {
      if ( there is data left in readInfo ) {
         copy to userBuf;
     } else {
       RETRY_LOOP {
           in.readChunk().
           in.readChecksum();
          compare the checksum
          If there are mismatches, in.seekToNewSource() etc
      }
  }
}

resetReadInfo() { readInfo = 0; }

}
{code}

Hope this makes it clear where InputChecker I have in mind fits in. Note that here FSInputChecker
 in ChecksumFileSystem and DFSInputStream in DFS, implement readChunk() and reacChecksum()
internally.


> 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
>
>
> 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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