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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 Thu, 07 Jun 2007 15:37:26 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-1470?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12502388
] 

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


readBuffer.java is tested in my dev environment. Checksumming closest to User I think can
be solved independently. I agree that it is very important and will be solved for HADOOP-1134.


Some more considerations for InputChecker design:

If purpose of this Jira is to provide a generic InputChecker, I wonder why 2 streams (where
it imposes that every checksum block be of the same size, for eg.) is very generic. Does it
support different types of checksums (DFS already uses two types)? Are we confident this serves
future FS well? Or is it ok to modify InputChecker (probably all the existing FS'es) then?


When InputChecker reads  4k (say by some equivalent of readFully()), should DFS necessarily
read 256 MB  of block data in the case where bytesPerChecksum is 64K to solve that? Of course
no. This is just one example of various things DFS support for this InputChecker needs to
handle.

I have probably thought more than I should about this :). Back to HADOOP-1134.


> 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
>
>
> 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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