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From "Sameer Paranjpye (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 Mon, 11 Jun 2007 22:45:26 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-1470?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12503663

Sameer Paranjpye commented on HADOOP-1470:

>From what I've seen of the implementations of FSInputChecker and FSOutputSummer it looks
like they can be very simply tweaked to handle the two cases of

a) parallel data and checksum streams and
b) a single stream with data and checksums interleaved

they can already handle the end-of-stream case where the number of bytes to be checksummed
is <= bytesPerChecksum

why not just tweak the existing implementations and make them capable of handling a single
stream with data and checksums interleaved?

The implementation could then be used by both ChecksumFilesystem the DistributedFilesystem.

DFSInputStream would then instantiate a DFSBlockStream (a new class) wrap it in an FSInputChecker
and commence reading, it would re-initialize the input checker with a new DFSBlockStream when
the end of a block was reached or when seek was called, the 'pread' case would need to be
handled carefully.

I don't see the need for methods to read data and checksums independently of each other.

> 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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