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From "M. C. Srivas (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-2699) Store data and checksums together in block file
Date Mon, 19 Dec 2011 07:02:30 GMT

    [ https://issues.apache.org/jira/browse/HDFS-2699?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13172059#comment-13172059
] 

M. C. Srivas commented on HDFS-2699:
------------------------------------

@Todd: no one is arguing that putting the CRC inline is not beneficial wrt seek time. Recalculating
CRC over with a 4K block is substantially slower than with a 512-byte block (256 bytes vs
2K on the average is a 10x factor). Imagine appending continuously to the HBase WAL with the
128-byte records that you mentioned in another thread ... the CPU burn will be much worse
with 4K CRC blocks.

Secondly, the disk manufacturers guarantee only a 512-byte atomicity on disk. Linux doing
a 4K block write guarantees almost nothing wrt atomicity of that 4K write to disk. On a crash,
unless you are running some sort of RAID or data-journal, there is a likelihood of the 4K
block that's in-flight getting corrupted.

                
> 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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