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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, 19 Jun 2007 19:51:26 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-1470?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12506279
] 

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

Chatted with Sameer regd preads in DFS, and this is what he has in mind :

DFSInputStream() opens on InputChecker for each block it is reading from. So normally there
would be one 'blockStream'. In case of pread, instead of creating a new DFSInputStream as
I was thinking, it would create one InputChecker with each block involved (in sequence, it
could even fetch data parallely in future). So pread() will not create a new DFSInputStream().
This will use all the Block cache etc. This looks good to me. The current Block Level CRCs
has similar structure (it creates one {{BlockStream}} object for each block etc).

Also above pread implementation is idependent of actual implementation of InputChecker. ie
it is independent of whether InputChecker uses 'readChunk()', 'readChecksum()' or 'two streams'.


> 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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