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From "Hairong Kuang (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 Thu, 14 Jun 2007 05:30:27 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-1470?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12504542

Hairong Kuang commented on HADOOP-1470:

Can not stop thinking because I feel the same as Doug that seek could be generialized. :-)

It looks like the current proposed FSInputChecker could be broken into two classes:

1. ChecksumFSInputStream is an abstract of datas and checksums or an interleaved stream as
in DFS.  ChecksumFSInputStream extends FSInputStream and has checksum-related abstract methods:
readChunk, readChecksum, getChunkBoundary, nextChunkSize, seekToNewSource, and reportChecksumError

2. The generic checker FSInputChecker is a concrete class. It contains a ChecksumFSInputStream,
extends FSInputStream, and provides generic implementation of methods: read, seek, skip etc.

Every checksum file system should implement ChecksumFSInputStream, while every non checksum
file system implements FSInputStream. So  FSDataInputStreams contains a FSInputChecker for
checksum file systems or a FSInputStream for non checksumed file system. 

I am still not clear how this approach supports change of Checksum between blocks. Any thoughts?

> 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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