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From "Hari Mankude (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-2699) Store data and checksums together in block file
Date Tue, 20 Dec 2011 18:46:31 GMT

    [ https://issues.apache.org/jira/browse/HDFS-2699?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13173399#comment-13173399
] 

Hari Mankude commented on HDFS-2699:
------------------------------------

One of the issues with inlining checksums (512 bytes of data + 4 byte checksum) is that it
would application layer slower since applications might have to copy data to get contiguous
data only chunks. One of the tradeoff is between checksum disk iops and application copies
on the read side. Similarly, writes will have to inline checksum bytes in the data stream
buffers.
                
> Store data and checksums together in block file
> -----------------------------------------------
>
>                 Key: HDFS-2699
>                 URL: https://issues.apache.org/jira/browse/HDFS-2699
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>            Reporter: dhruba borthakur
>            Assignee: dhruba borthakur
>
> The current implementation of HDFS stores the data in one block file and the metadata(checksum)
in another block file. This means that every read from HDFS actually consumes two disk iops,
one to the datafile and one to the checksum file. This is a major problem for scaling HBase,
because HBase is usually  bottlenecked on the number of random disk iops that the storage-hardware
offers.

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