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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 Fri, 29 Jun 2007 18:58:05 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-1470?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12509177

Hairong Kuang commented on HADOOP-1470:

> It might be easiest to implement this as two buffers: one, multi-chunk and un-checksummed
This proposal sounds good to me.

But I am not fully convinced if we should support buffering in FSInputChecker or not. It seems
to me that Sameer wants readChunk to support both pread & read in FSInputChecker. It's
not clear to me if we want to provide a generic pread in FSInputChecker. If yes, I am not
sure if having one common API readChunk to implement both pread and read is a good idea. If
readChunk is for supporting only read, then we could have readChunk to be optimized for sequential
reading. We should not need to worry about mutiple preads causing excessive seeks.

> 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,
> 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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