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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 Wed, 13 Jun 2007 21:55:26 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-1470?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12504449
] 

Hairong Kuang commented on HADOOP-1470:
---------------------------------------

> readAvailable() gives the valid data left in InputChecker's buffer. So that getPos()
will be {{ datas.getPos() - readAvailable() }}.

If this is the only usage, I am not sure if readAvailable() is really needed.  Caching the
current pos is an alternative way of implementing getPos(), which could be done in the generic
InputChecker.

> Yes. ChecksumFS needs this to retain current functionality of "stopSumming()" (with InputChecker
it would do 'setChecksum(null)'). 
stopSumming is internal to InputChecker so ChecksumFS does not need method setChecksum. But
I am not clear if DFS needs it.

> right. not strictly needed because InputChecker should gaurantee buf has enough space
for next chunk. It is the common way to give 'len' everywhere else where we read.
Having paremter "len" requires the implementation of readChunk to check if it is valid. It
has no other use.


> 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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